Chances are, if there's a known cheater on the leaderboard... we already know about it. We have a number of potential enforcement actions that we can take, some of which may be visible to you and some of which are not. It may be that we're trying to convince him to stop cheating and come to the side of Right, Truth, and Unicorns. It could be that we're conducting our own research and we're watching him to learn what new tricks he has. Banning him immediately would deprive us of that knowledge.
Either way, know this: our trust and safety team is very, very good. They've recovered millions of fraudulently earned points and prevented payouts to many fake accounts. And even when the fraudsters manage to collect, it's not because we don't know - it's because we're watching and learning. And it turns out, most of them are really bad at being fraudsters.
Of course, we could also be leaving their accounts active because we love seeing them think they're getting one over on us and just can't wait to get the emails wondering where their token payouts are at the end of the season. :D . We're sneaky like that.