Well YOU should report him to a Mod if it's really bad stuff, or if there's an insane amount. Flag his comments that you see, and any other spam comments that you stumble upon.
You can't stop spammers as a whole, you can only get rid of the ones that you find to get some peace before the next one starts causing chaos, unless they're a small-time spammer, then you're just flicking a single bee off your flower.
I think it is impossible to stop spamming completly but if you put a notice up warning spammers that something will happen if they do spam it could prevent it. That is my suggestion.
Maybe there should be a SPAM SQUAD where there are specific people who look out for them and they could have the ability to report a profile or something to that effect I haven't thought it through that much really.