One thing I noticed is that a lot of serial downvoters never post content and are therefore immune to counterattack in that way.

The only way I can see to "hurt them back" would be to downvote everything they upvote, proportionately of course, basically deleting all their votes, and if the counterattack account only had 25 rep, it would not inflict undue rep damage to the "innocent bystanders".

bloom is the example that springs to mind.