That's actually better than if the threads were stripped. There are several options:

Buy a bolt extractor, drill out the bolt and remove it with the extractor, often called an EZ-out.

Drill off the head of the bolt, remove the crank with an extractor and remove the remainder of the bolt with a vice-grip.

Cut a slot in the head of the bolt with a cutoff tool, use an impact driver to remove the bolt.

If there's anything left of the hex at all, pound an SAE hex wrench into the bolt and try removing the bolt again.

Try a Torx wrench. They can often fit more tightly in a stripped hex.

Cut off the crankarm.