Different usernames posting, but exactly the same ad text every time.
One possible defense: If a post gets more than X times, identify some text in the ad, and if it appears in further chat messages, just don't let them be seen by anyone but the spammer. They'll eventually discover the block, and then they'll have to try and figure out what the trigger text is, only to get blocked again for reasons unknown to them. Eventually, they'll run out of ways to say what they want.
Just kicking them off means they'll just create another level 1 character and continue spamming the same crap.