As Dallas say's, its an option (not something I agree with, but there ya go). If he doe's send abusive pm's, you also have the option to report them to HTC. Do so, if it makes him stop, so much the better. If it doesn't make him stop, at least he'll lose his chat privileges for a while.
Dogfite, whilst I dont agree with your decision not to do something about his pm'ing, I get where your coming from.
Wurzel