Technically, there's nothing I can think of you can do.
Socially, there's a lot. For example, all of the online board game servers I've seen make very public the user's win/loss record, and compute the user's rank from that. Doesn't that just encourage people to want to win? Instead, I'd record all games, but not present a win/loss record anywhere (does anybody at a real chess tournament know how many games they've won/lost ever?). Make rank a user-entered number, used for the purposes of finding an appropriate partner only, so simply showing rating of 5000 is meaningless. If you need to have some kind of 'user rating', then add a commentary system, to let users comment on moves of other people's games, and then let other users rate the comments. Commentary is one thing I haven't seen computers do intelligently yet, so it's something you can probably assume comes from a real person.