You will want to go to the Advanced Tab in your Plays.tv Preferences:
Scroll down until you get to "Manage Applications":
The list with populated game exes is your "Whitelist" which allows these programs to record with Plays.tv. Clicking on the "Blacklist" button will place it in the lower list of excluded applications. Once a game is placed there, it will not record.
You can bring it back to the Detected Games list by clicking the "Restore" button.