Griefing is the act of ruining large or very noticeable portions of other people's creations on a multiplayer Creative or Survival server. It is frowned upon by the majority of Minecraft users because it ruins many hours of work and makes maps untidy. In best case scenarios, the griefer is quickly removed from the server and their damage is trivial to repair. At worst, griefing can destroy an entire server if it does not have backups available.
The most effective way to fight griefers is to have the Admin or an Op regularly monitor players by using the teleport Op command. If a griefer is found, a /kick or /ban command is usually used. Sometimes, a griefer can change their name to evade the ban. If this occurs, an IP ban can be used on the perpetrator's IP address, but griefers can sometimes get past this ban as well by using a proxy to change their IP. When this is the case, it is probably best to create a whitelist for all trusted players or to shut down the server until the griefer gets bored and moves on. A common way to keep griefers from entering a server while no Ops are online is to use Spawn Jails. If built properly, they will ensure no player will be able to enter the map unless an Op lets them. However, some players consider this a rude practice.