As I mentioned in Will Filters Kill Spam?, following all the urls in a spam would have an amusing side-effect. If popular email clients did this in order to filter spam, the spammer's servers would take a serious pounding. The more I think about this, the better an idea it seems. This isn't just amusing; it would be hard to imagine a more perfectly targeted counterattack on spammers.
So I'd like to suggest an additional feature to those working on spam filters: a "punish" mode which, if turned on, would retrieve whatever's at the end of every url in a suspected spam n times, where n could be set by the user.
There's one catch - how many ISP's are going to see your response as a hostile net attack - and get you in hot water over it? The motivation for this is fine, and I have no problem whatsoever with pounding the servers of the spam supporters - but your ISP may well not be so understanding