You can hacksaw a slot across the screw head and use a screwdriver to remove.
You can glue an allen key into the hole leave to set and hope to hell it comes out.
You can use a centrepunch to remove.
If all else fails hit it with a big hammer and hope the screw falls out on its own free will*.
*under no circumstances must you try this without first consulting your common sense**.
**If lacking common sense put the hammer down and step away from the pointy objects