This is a new one for us.
We make it fairly easy to find your posts http://forums.theregister.co.uk/user/4800/
Login, load that page, run down it clicking.
So - activity is not robotic. every day or so, someone spends ~5 minutes downvoting 10-30 posts, then goes away and comes back a few hours/days later and does the same thing.
As this is a new one for us, we have no traffic lights in place.
We could perhaps set up a daily/weekly report on voting activity and then we deal with it via humans, not tech.
What do you guys think?