CouchCast: Help! There's a Clingy/Obsessive Guy in My Friend Group.

Have you ever had to deal with a guy who just didn't seem to get the hint? Maybe he's asked you out several times and you always tell him no, but he still won't let it go? That's the situation we're dealing with today. Ideas for being direct, but not rude, when to involve other people, and when to just cut off contact entirely.