Well, I had to do it manually:
1. open phpmyadmin, find userid in qa_users
2. go to table qa_uservotes, find userid (SEARCH LIKE) and delete all votes
3. >Admin >Stats do [Recount Posts] and then [Recalculate User Points]
I asked myself: How do you remove all votes of a certain user? By deleting him?
If you know an answer, let us know :)
If not, we need a plugin for this for sure =)