Other options after exhausting the ones posted above are to do your best to make sure your paid version comes up first in the search engines and perhaps release a new version (even if it's totally cosmetic and in name only) to make the free bittorrent one look like the "old outdated" version. Also the free version may not eat into your business as the bittorrent crowd may not be your target audience anyway.

Hope it works out for your client.