On a wiki with adequate revision control, unwanted content can be selectively removed. A common pattern is to be permissive in editing content, but to be restrictive in keeping it. Any content that is clearly not acceptable is rolled back or reverted to the previously edited version. This may become an adoption anti-pattern if the rollback is performed too much, however.
Roll back unwanted content that has been added. The trick is that the underlying wiki must support adequately flexible and precise revision control.
Wikipedia does a great deal of this.