Google suggests adding a rel="nofollow" attribute to links which you don’t want Google to follow. They suggest this in order to reduce the PageRank incentive for comment spam. If PageRank incentives were all there were to the problem, this might have a snowball’s chance in Hell, but they aren’t, so this won’t. The spambots, like more respectable advertisers, don’t care about PageRank, except as a means to a click. If the links remain, the clicks will come.
Public floggings are the answer, not mere technical impediments.