Saw a slot into the bolt head and remove it with a screwdriver if you've given up conservative solutions already. Trouble is, then you're losing that spacer barrel thigy... So you should try hammering an imperial allen bit, a torx bit or a screwdriver of slightly too large size into that motherlover. My pick would be a one size too large torx bit, filed slightly to a wedge shape. Pick something that almost fits, bash it in w/ a big hammer and turn.