The majority of your spam posts will be an attempt to post urls on your pages... preg_match or strpos the incoming information...
look for phpBB code and html anchor tags...
That will drop your spma posts significantly without having to resort to captcha... if you go with captcha... be aware that you should design your own or at the very least go with a good one... people are determined to break the captcha mechanism... look at how much harder Google captcha's have recently become...
be aware that blocking ips and proxies will essentially at one point or another block access to legitimate users...
Breaking a Visual CAPTCHA