In order to prevent cheating peer in closed tracker, there is a very easy solution to solve this problem. However, it might be too strick.
Problem: Some clients do not even try to contact tracker but they are still able to leech using historical peers information.
Solution: Count amount of download by size of the torrent immediately when download its metadata
Advantage: Disable cheating of unreporting clients
Disadvantage: Downloading only some files may not fair