Not for nothing, but tagging as many bounty hunters as humanly possible was essentially what actmyname (whatever happened to him) and I were doing just prior to the merit system being introduced, as a way to discourage shitposting.  You'll remember at the time that there weren't any effective tools for cleaning up the forum, so the trust system was all we had. 

Theymos has already given his opinion on how negative trust should be handed out, and I don't think he'd approve of gang-tagging bounty hunters indiscriminately.  Granted, I'd be more than happy to join in on such a tagging party, but 1) it goes against the guidance Theymos has given, and 2) as you said, it would probably be ineffective.  It might be a little bit effective, but I seriously doubt it would discourage many of these hardcore bounty participants, and it'd just be a pain in the ass for whoever decided to do the tagging.

Then there's that.  And it all comes down to how the trust system should be used.  You really shouldn't just give negative trust to a bounty hunter simply because that's what they're doing on the forum.  I get that they're spammers (not just on bitcointalk, as you pointed out), but red trust should be reserved for truly untrustworthy members.