Trolling

Griefing is frowned upon by the majority of Minecraft users. It wrecks construction, ruins many hours of work, and makes levels untidy. At best, griefing annoys other players, at worst it can destroy an entire server.

Random Destruction
This is when a griefer destroys random blocks, putting holes in walls and other structures, making them look broken and messy. The way to combat this is to just repair the damage and have an admin kick the offender, then repair the damage by hand. If there is no admin to kick, then just wait for the griefer to get bored.

Random Construction
This is when a griefer creates random blocks on constructions, making them look messy. The way to combat this is to just remove the extra blocks and have an admin kick the offender. If there is no admin to kick, again, just wait for the griefer to get bored. This type of griefing doesn't seem as bad as the first, but it can be just as annoying. Often griefers both destroy and create blocks in their quest to annoy others.

Water Flood
This occurs when a griefer destroys blocks causing water to spill across the level and into/over any contructions players have made. This normally can only happen when the construction is underground, or the ground level is below the level of the ocean. There have also been instances of hackers making water blocks in the middle of the sky, causing it to flood the entire level. If the damage is minimal, or the area small, a player can fill the area with another type of block, such as leaves, then dig them out again, leaving the area dry, or use the sponge block. Any water nearby will reflood the empty area again however. Another danger that water also causes is the huge number of chunk updates that occur when water spreads rapidly can cause buffers to fill and players to lose connection to the server.

Lava Flood
Similar to a Water Flood, a lava floods the area with lava instead of water. Lava is far slower than water, so most of the time, if it's caught the lava can be stopped with a few well placed blocks. Lava floods are also rarer because lava is mainly found underground, at depths far below the common areas of water. However, well-planned lava floods can be catastrophic, because sponge blocks do not absorb lava, so the area must be cleaned out by hand by displacing the lava with blocks and erasing them, drying off the area.

Offensive Constructions
Some griefers often grief others by writing rude words or drawing phallic symbols everywhere. It's often generally harmless fun, but if a player finds it offensive they can destroy the offending problem. This is far less of a problem that the others, although some constructions high up in the sky may take some creative thinking to remove.

Flattening
A relatively new and rare form of griefing but still worth mentioning. Flattening is exactly what it sounds like, destroying (or in some cases adding) blocks until the ground in an area is completely flat. While random destruction can cause a level to look ugly, flattening completely removes the structure from existence as if it were never there. This form of griefing is much harder to perform and is usually done while servers are empty or have no admins present. The best ways to defend against this kind of griefing is the use of spawn prisons (although these are generally ineffectual) or shutting the server down when it can not be watched.

Tools of The Griefer
This is a catalog of the different methods or programs used by griefers to perform the aforementioned tasks.

Hacks
Programs that manipulate Minecraft in a way that was not originally intended by Notch.
 * flying - Now in widespread use by both griefers and admins, this is a common hack and can allow griefers to easily reach tall structures and begin deconstruction of areas that will be difficult to reach by players lacking this hack.
 * auto-click - The staple tool of any griefer is the auto-click which allows the player to quickly and messily place or delete blocks. When combined with increased speed this can be a destructive combination.
 * noclip - This means to turn off all collision with other blocks, usually used to bypass unbreakable stone. This is done in several ways but it is generally used solely to bypass unbreakable stone. This ability is very common.

Social Engineering
Not used by most greifers, but if done correctly it can be extremely confusing to figure out who the griefer is among a server full of people. Social Engineering, in the context of griefing, is convincing admins or other players that you have good intentions and are to be trusted. While most griefers use more of a guerrilla strategy, this sort of griefer is significantly more dangerous because of the confusion they can cause.

Time
Some griefers will wait in a server or return to it when there are no admins online, at which point they could destroy until another admin logs in. It can be very stressful to regular players when this occurs as all they can do is watch as their creation is destroyed before their eyes and may cause players to lose faith in a server they frequent.

Bots
The most destructive tool a griefer can ever employ. Very uncommon and they always come in swarms. Not much is known about exactly how they work or their full capabilities but these bots are very dangerous and are generally only employed if a specific server has been "targeted". The bots can make very human like responses and it is suspected that if the bot swarm is being overseen by a hacker, the hacker can speak for the bots when they are questioned. These bots come complete with many macros which can instantaneously create pyramids or massive holes in any structure; they also fly around and employ random destruction greifing. IP banning and kicking are useless as the bots switch IPs and account names in seconds and come right back, the only real defense is to shut down the server and wait. Some of the behaviors bots have been seen to exhibit are:
 * Not speaking unless spoken too (will not ask to be let out of spawn prison)
 * using long, misleading names that are difficult to type
 * frequent jumping (although this is exhibit by several human players as well)
 * rotating head all around to check for other players, usually done immediately before they begin to grief
 * No clear purpose or destination, they wander.

Combatting Griefers
The best way to combat griefers is to kick and ban them, often with a permanent ip ban. Until there is a more robust system, however, players and admins just need to be vigilant. When a griefer is found, an admin can verify what they are doing by teleporting to them. After that a /kick or /ban command will remove the griefer from the server.

If an admin is not on the server, or is handling a heavy load of griefers, a good strategy for players is to ignore the griefer. As hard as it is to ignore somebody who is ruining your hard work, Griefers thrive on the annoyance of others. Oftentimes simply ignoring a griefer will cause them to get bored and leave. This would prove very ineffectual and it is very unlikely griefers would "get bored" rather they would partake in more large scale destruction such as flattening. The actual best way to handle the situation is to rebuild things as the griefer destroys them. this serves the dual purpose of repairing damage as well as causing anger to the greifer by effectively undoing what they have undone, a taste of their own medicine so to speak.

Griefer swarms
While one griefer can be bad, on rare occasions they may attack in groups. Sometimes members get to the point of blaming others in the group to avoid being caught. They can also attack rapidly, typically abusing votebans to clear out the server. A typical sign is about four people(four is a common voteban threshold) people joining at once, but that can be coincidence.

The swarm also provides the problem of being harder to ban.