Unfortunately, you only have to compete with real users using that specific feature, and there aren't that many when measured against the power of even cheap attacks.
Then again:
1. even enabling personal blacklists would be a huge benefit to power users even if they aren't aggregated.
2. the user I replied to above suggested running n 1000 vps-es or using a criminal botnet. Both of these are significant hurdles.
Also while running n 1000 vps-es isn't a significant hurdle in todays clouds if it is a one time job, it at least becomes expensive if the blacklists are generated based on active users at random times around the clock forcing would-be abusers to not only fire up their vps-es, running a couple of queries and shut them down but also to keep them running around the clock, or rather in 8 hour shifts to not trigger abuse detection.
Price. You have to be seriously dedicated or have serious economic muscles to pull this off.