Not a whole ton you can do other than drill it out. That shouldn't be too hard. Just be sure to use a bit that is somewhat smaller in diameter than the screw.
While I haven't attempted this myself, Sheldon Brown says that you can ride the 20 without using the locking bolt. Depending on how you ride, that may be possible.
That being said, I'd drill it out and use a bolt that fits.
Next time, if it doesn't move with medium applied force, WD40 it and let it sit for a while before trying again.