Stuff the bugger in a bag of holding.
May not be dead, but he's out of your hair, and, with how annoyed he'd be, you could sic him on the next enemy you faced that you had some trouble with.
And, assuming it to be a dragon, your problem is solved.
...Oh, wait, Mr Welsch's list forbids this...Online list of well over a thousand ridiculous things that shouldn't be allowed in a roleplaying game. Includes that the demi-lich only falls for being stuffed in the bag of holding once, and that 'search the castle' doesn't mean 'shell it with artillery, and sift through the rubble.'
Either way, no sense in clobbering such a potentially useful resource.