[JoeBOBBillyTed]

Here are a couple of ideas.

#1 change the questions in the to be something that is more difficult to guess, and change the questions often. Try something like What is larger a bus or a car. Don't do things like math questions as these can be created.

#2 Make sure you require email validation, some of these bots do not do email validations

#3 For new users, put there messages in a queue that you have to approve. This will not stop the problem, but will hide it from non-staff.

#4 Check for patterns in the registrations, for instance I had an issue with someone and they put TEST in some fields, this could be stopped by blocking registrations with TEST in any field.

#5 Require that accounts be authorized to post

#6 There is a hack that allows for you to prevent people from posting links / images until so many posts. Install this and set to a low number like 4-5.

#7 add their urls to bad word filter

That is a good start, if you do that, they will give up.