Forums and blogs are susceptible to post spam because most are large opensource or commercial scripts. The fact that there are thousands identical scripts makes them a prime target to spammers. The trick to prevent forum spam is to confuse the spam bots. Most all bots will fail to register a user if given something unexpected. For forums I administer, I wrote a vbulletin mod that requires a bit of human intuition to solve (not much mind you) such as have a text input field that must remain blank. This simple measure has prevented nearly 99% of forum post spam.