The bolt is probably rusted to the inner sleeve of the rubber bushing. Dousing the whole shebang with a good penetrating oil, then using a hammer and drift to drive the bolt out, is the way that I'd approach this task.

I use Tri-Flow, but Kroil has a good reputation too.

Heat would not be a good idea in this case.

Lastly, I'd use a Dremel or equivalent to remove the bolt head and drive the bolt out that direction, if you can't get sufficient room to drive it out the normal way. Because you're replacing the a-arm, you don't have to worry about deforming the arm's end, and so you have more options available.

Regards,
Al S.