I've seen several comments here and other places that people wished they could mute players. I didn't think you could either until I found out how by accident yesterday... Maybe everyone already knew, but if not, here's how:
Once the game has started, hit your select button. This as you know brings up the list of players currently in the game with you (shows k/d's and points etc.)
Use the directional pad (up/down) to highlight the offending noisy player, and hit X.
Viola... they are muted.
If you don't have a headset and want to mute everyone so you can hear the game or maybe just your buddy, just rinse and repeat.
Personally I just leave my headset draped over the ps3 muted. I have a Sony 5.1 surround sound system and I make use of it while playing COD4
