I dunno...I'm missing something here. You're getting bent out of shape over a Yahoo Groups Administrator who is threatening to turn you into a collections agency for $9?
You're claiming he's threatening you, yet you're not telling us how he's threatened you. Personally, I don't consider him telling you he's going to turn you into a collections agency over $9 a threat. I consider that showing you his total ignorance and stupidity for the whole situation.
Bottom line is this. The world is full of idiots, and unfortunately, the majority of those idiots waste their life on the internet trying to make other people's lives hell. As long as morons like this are allowed to affect people the way this moron's affecting you, they'll continue. You have several ways of avoiding this person's e-mail, including just not reading the things. You are subjecting yourself to this idiot by continuing to read his e-mails and leting him affect you, hence he's succeeding in what he wants to accomplish.
The solution is simple. Ignore his e-mails and move on. It doesn't get any easier than that.