Does your email spam filter pick up on those posts when they're emailed to you, and mark them as spam? If so, it would probably be possible to write a simple script that would automatically go and delete the comment based on the spam score it received? Unfortunately, even if you're not talking to a spambot, you're talking to someone stupid enough to think spamming your site is going to be constructive, and so your post probably isn't going to get through to them anyway :-(