Strange website problem

I have a fireworks forum as part of my fireworks-zone website.

Sadly some **** keeps using it to advertise Russian pron sites and other crap.
The forum has full IP tracking and member banning etc but this **** keeps getting round it.

When I check the web logs their is no mention of any details it just says the IP address is withheld.

I need to get rid of this **** very fast as its the main time for fireworks websites and this is making my site get a bad reputation etc.

Any ideas please ?