Report,Delete,Move on and I know sometimes I get those message where they invite me to a party and most of the time I don't accept because they usually use foul language and just end up kicking me from the party. So I just ignore the message of coarse report them because no one should be the victim of those hurtful players because they lost or hate how you played the game.
It's weird, I rarely get nasty messages - but I used to all the time in Halo 3 and Reach. I'm pretty bad at Halo (always have been) so my gameplay usually tics people off pretty royally, but lately . . . nothing.
I always just Jehova's Witness them. I pretend to act like an overly concerned die-hard Christian trying to save them from the one true God's heavenly wrath that he will smite down upon you, if you don't get down on your knees and repent for your sins. Hey, look, I'll keep you in my prayers, you poor, lost, soul, you. :((((
It's extremely hard to, if not impossible to please everyone. If someone is being a bummer or d***, just know that there other people that enjoy you just as you are. Perhaps time will help us to recover to those days of simply having fun.