Sometimes users repeatedly post content that they should not, like words you'd rather not see on the board, or links to websites to which you don't want to link. Under Admin Control Panel -> Spam Blocking you can set up patterns (regular expression, actually) that should be prevented from being posted. The pattern is case-insensitive, so "porn" would match both "porn" and "Porn". In the case of blocking web sites from being linked, you can simply use the domain name. That wouldn't prevent the IP address from being used instead, but if a user goes to such length, it may be time for a chat with him/her...