The G-Chatter. If your day is spent dodging IMs about her latest diet when she bailed on that reference you needed her to write, don't just block her screen name. Businesses have performance standards, and the same goes for friendships. Enumerate grievances about how she's failed you.
The Facebook Stalker. She could mistake your de-friending as a computer glitch instead of a purposeful deduction from your friendship payroll. Being straightforward is important. The first time I handled mass layoffs, I practiced with a colleague. After I finished my spiel, he said, "I still have a job, right?" I had tried to soften the blow so much that I hadn't been clear.
The Casual Encounter. A good boss will fire the intern with the same professionalism she'd use on a veteran employee. If you've had it with your running buddy, tell her that you've decided to go in another direction — and hand her the name of another runners' group she could try.