what's the protocol for telling someone you met on an online dating site that you're not interested?

[another1inneed]

actually, what's the protocol for telling anyone in the world that you're not interested. i actually don't think i've ever done that successfully, and the few times i remember trying were fairly disastrous. i usually just ignore the person til they go away, but then sometimes they don't.

suggestions? advice? informative anecdotes?