There are 3 ways
1) Add words to the Censor (found in the Configuration under Miscellaneous). This will still allow the post while putting a replacement text over the word.
2) Use the SpamX plugin. Edit your personal blacklist and add words you do not want to appear in a post. If they do the post will be marked as spam and not saved.
3) Human Moderation. Setup a sticky topic in the forum with some rules. Tell users if they violate he rules they will be banned from the forum or site.
One of the Geeklog Core Developers.