Easiest way, you make them sign an agreement during peace negotiations. Maybe even just demonstrating that you are serious and what the consequences are might a

Easiest way, you make them sign an agreement during peace negotiations. Maybe even just demonstrating that you are serious and what the consequences are might act as a deterant for at least some time. Install a pro Russian government. You are right, nothing will guarantee that they never become a NATO member, but for some time is a plausible goal. Even just being in an active conflict prevents a NATO membership unless the rules are changed which is what Crimea and eastern Ukraine achieved since 2014.