You could perhaps nullify the upvotes after a few months to preserve the system in the long run. The idea is that the short-term effect of his actions should be net-negative from his own perspective.
Other ways of achieving the same effect may also work; that was just what I came up with after five minutes of thinking about it.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.