One of my ex girlfriends had a roommate she didn't always get along with. He would use her phone because it was cordless and he could wander around the house with it. And he wouldn't return it to the base. She'd constantly get irritated about it. So she hid the phone from him. But invariably she'd leave it where he could find it. And he wouldn't stop even though she asked. Yup. He was an ass. But I had a conversation with her once, went something like this. "Why don't you just buy him a cheap cordless phone?" "Why do I have to buy him a phone to get him to stop using mine?" "Well, consider it an investment in your serenity." "But then he'd win!" "Okay, how about I buy him a cordless phone." "If you do, I'll be pissed at you."
So the question comes down to, would you rather be right, or happy? Most of the time, you don't get to be both.