As bitznpcz said, just don't port forward to that pc. On your router settings, under applications settings (or whatever it's called), you can give permission to certain applications to use a specific port. Assign the ports for the Utorrent application for only the IPs of those two systems you'd like Utorrent to run on. Remember that you should have a static IP address on each system.
My guess is that it would still be able to download stuff, but it will me very limited as the port would be blocked by the router. At this statement, I'd like to ask for the experienced techies here, why does the application still download (at slow speeds), even if the specific port is blocked on the router.