If someone takes the time to downvote 50 questions per day, I'm pretty sure they have a reason, right? Nobody would take that time without thinking they're making progress towards some kind of objective.
Now, if you consider those downvotes go against your forum rules, then you should block or even delete the user. If you penalize the user, you get rid of the source of the issue. Removing 50 downvotes per day, even if it was possible, you're just adding more daily work for yourself.