There are a few ways to remove bound/sheared screws.
Firstly, try and cut a groove in the head with a hacksaw or dremel-type cutting disc, and get a flat head on it.
Failing this get a pair of mole-grips (uber-pliers) and get them as tight as you possibly can around the head then spin it out.
Then we bring out the big guns:
Drilling: You can drill off the head (or grind it off) inorder to seperate the surfaces being screwed together. This is good if you don't want to use the screw hole again.
Pulling: once you've drilled/ground the head off, you take care to drill a small hole down the centre of the screw. You can purchase 'taps' (which cut screw threads in metal) which are threaded backwards ('pullers') you basically screw one of these down the hole you've drilled until it bites then keep turning and it'll unscrew the screw shank.
We use these methods on aricraft (nearly all the screws are buggered after a few hours flying in icing conditions) and I can testify to the effectiveness of them. The drilling/pulling method takes much less time that it seems, and once you've done it once it's simple to repeat.
Good luck!