Question of the Day: What would you do if your best friend's partner came onto you?

This was a suggested question for me today and I have to say, it gets brought up often. While it's disappointing to think that significant others hit on their mate's best friend's, it does happen. So what should one do as the recipient of untoward advances? Do you approach your BFF and tell them? Do you tell their partner they better tell their S/O or else you will? If they're a Pamela Andersson or George Clooney clone, do you say, "F#!@ it", throw caution to the wind, go for it and hope it stays on the Q.T.? What is the proper protocol here?