I have quite a lot of non-gambling forums and they have historically always been targetted by such spammers, even ones with hardly any members and no posts for months. Yet my main casino/poker forum with nearly 5000 members has never been spammed, weird eh.
Anyway I took precautions and got someone from http://www.phpbb.com/phpBB/ to put in a whole anti-spammer control panel with many great features. They will usually do it on a volunteer basis and then you can make a donation. Depends if you use phpbb or not of course though :1circling