I second the notion of not promptly replying to his e-mails.
He likes the instant feedback, he's living for it. Respond to his e-mails ... but wait three or four days. Or reply only on Saturdays if you've got a day job.
As far as reining him in -- is he actually
creating a problem? What do the other members think of him?
Might be that he's worth keeping simply for the free enthusiasm. Set some ground rules and stick to them but you might put him in charge of something where he can't do too much damage, such making him a mod and putting him to work on the task of spam deletion (set VERY clear ground rules on what constitutes spam) or maintaining a list of events. Put his energy to work ... *shrug* He probably just wants to look important.
Just watch him closely to make sure he's not upsetting the other forum members or taking liberties such as representing himself as the board owner, or deleting posts he doesn't like, or luring your members off to another board he likes better.
-- The latter's a real possibility, I've seen it happen. If he's busy and popular and everyone knows him, and he gets offended, he might head elsewhere. And half your board could go with him.