About 9 months ago my friend's band had to let go of their singer because the guy was having a number of problems, both musically and personally, and it was severely damaging the morale of the band, as well as holding them back creatively... So they did the logical thing and let him go.

As you can guess, he took it pretty hard and it didn't take too long before he was trashing my friend's band at every opportunity, even going as far as sending lot's of flaming emails and defacing the band's guestbook, over and over...

My good friend, who is the drummer in this band finally had some talks with the guy and thought everything was cool (actually he tried to iron things out with the guy a number of times), but every few weeks or so the guy would start feeling unhappy and start shit once again. Things have been quiet for a few months until my friend got another long email today... Ugh, what a headache... the guy sort of implied physical threats this time as well. Keep in mind nobody in the band has talked to him (or even about him), in months.

Anybody else ever have these kind of problems? Any stories, ideas, etc.? Just curious.

Personally, no problems like that before.
I keep harassing my old Guitarist, Mr. Botts, though! He went off & became a "Suit & Tie" guy in the Corperate World and is avoiding me asking him to "Get the Band back together." Best damn Guitarist (besides Piggy) that i've heard and he has traded it all for a Suit and Office!
Oh well!
He gets paid better anyways.

We sacked our near-suicidal (and crap) guitar player but got the drummer to tell him.

Then we dumped the drummer by getting the bassist to tell him, then we dumped the bassist by getting the NEW drummer to tell him.

Unfortunatly, sick of all this duplicity, the singer left of his own accord.

You have no friends when you are in a band, pacts are made and broken behind your back. The only safe bet is taking the Mustaine route and assert your authority onto everyone else.