Our roommate won't stop flirting with me!

I moved in with my boyfriend about 3 months ago. He has a room-mate who lives in the furnished basement. Every time my boyfriend is not home, or not around, the room-mate flirts with me! He makes it so obvious that he likes me, and he has a girlfriend of 5 years. He even got me a birthday present - an expensive watch! My boyfriend has noticed the flirting but he doesn't know what to do either. This guy has go to lay off...help!
Our roommate won't stop flirting with me!
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