I'll offer the same advice I offered BobbiJo: the first thing you do is tell him explicitly that it didn't work out and you don't want him to contact you anymore. Those words: "Please don't contact me again. I won't reply to any communication."
This only works if you really don't reply, which sometimes takes more discipline than people think. It's also hard for most people to be explicit and not insert some polite "please and thank you" type words . . . which create mixed messages.
Keep in mind that when people talk about you sending mixed messages, what they really mean is that you left any kind of opening that someone could then apply the astounding power of wishful thinking and self-deception to, changing even frosty politeness into an invitation to keep trying.
