The WikiImmuneSystem usually works well.
* When a spammer puts a lot of links onto a page, someone who is online reverses it.
* When a newcomer erases one of the introductory pages, it is corrected within minutes.
* When two people apply this idea to each other's contributions, the result is an EditWar.
* Sometimes the result is complete rejection: either a HardBan or a WikiMindWipe.