Lots of ways:
1) Cut a deep slit through the middle of the bolt with a cutoff wheel and use a screwdriver.
2) Using a cutoff wheel, cut the bolt edges so it fits in the next size down socket.
3) Pound the next size down socket onto the bolt with a big fucking hammer.
4) Drill a hole in the middle and use an easy-out.
5) Buy special rounded and rusted nut removal sockets from the hardware store.
Whatever you do, spray some penetrating oil on it and let is sit for a bit before you remove it.