Just to help out and contribute to the many excellent responses, here's a list of a few additional options:
a) Just laugh. Hard.
b) "Scoreboard?" (The same response any Vol should give when UT is up 38-3 in the third quarter and a Vandy linebacker lays out a receiver and stands over him jumping up and down acting like he's made his first tackle ever)
c) "Whatever dude." (Usually inadequate and limited, but here, highly appropriate, best done while sadly shaking your head and walking away)
d) As many have said, you must make a bet. Money won is better than money earned, especially when won from a loud mouth trash-talking chump who has nothing to talk-trash about other than false bravado. Make the bet large and make it hurt. Betting UT over Vandy is much better than betting pocket aces.
Now, go show that guy back to his 2-10 locker room.