I agree with Poop on this, although I'd take it a step further and ban them for a month or two. Some people might think this is extreme, but people generally wise up and realize the error of their ways the first time they're banned for a month. And if they don't, it's nobody's problem but theirs, because they're not here to whine about it anymore.
Setting their post counts to 0 would be nice as well, but only as an addition to the ban, just to stress the matter and make sure they understand completely that spam will not be tolerated.