Annoying people...

What do you do when people are just simply annoying, yet are persistently wanting to hang out with you? Let me explain - annoying in the sense that they didn't do anything wrong, but their pure existance makes you want to put a pillow over your head and scream and you have to do everything in your power not to pop them one. Maybe they chew with their mouth open, or they tell jokes that just aren't funny, or worst yet they think they're the best thing since sliced bread...when they're just NOT. And...what if you've tried to 'ignore' them, but they just don't get it? What do you do???