That is on a bolt so if you just crank on it it will either break off or turn either way it is off. A pipe wrench put on square will take it off if it is already rounded if not then just crank on it with a good fitting 6 point socket and 1/2 drive ratchet. If all else fails you can grind it off. I would say heat but don't want to risk damage to the clear coat on the upper tree.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!