Erm...Move away from them? If you know they're pulling this stunt, increase your distance from them? That or flank them?
*sigh* Think people. There are tactics you can use to avoid getting cheap'd out.
Same with campers. You know walking through that area is going to get you killed by someone sitting, but you don't reckon by switching routes, flash, stun or smoke+thermal nading or just fragging the room you could knock them off their perch long enough for a kill or knife? Heck, get the FMJs out and go THROUGH THE WALL.
This is why I cry sometimes at FPSers (Including a friend of mine, by the way). Sometimes there would be an easy answer staring them in the face, neon signs and me spitting blood from my raw throat telling them the way unto Win-ness, only to receive insults back is priceless
Worth pointing out I haven't seen this yet. Cousin's got my copy until this weekend.