Are you sure its not threaded into the frame? if so usually a bit of pressure from the threaded end while someone else turns the bolt from the other end can get it shifting in the right direction
A good regular soak over a few days with a release oil can help if its stuck in the bearings or frame & always worth a try before getting drastic with it
You could get someone to support the bike while you give it a good sharp tap but leave the nut on loose so as not to damage the threads & use a block of wood as a punch, if it moves punch it back & forth a few times & use release oil every time, hopefully it will gradually move more each time until you can get it out
Having said all that my best guess is its stuck in the bearing spacer in the centre of the arm so heat wont help as you wont get enough heat into that to make a difference so unless anyone else has a bright idea you will have to cut it ..... last resort though
I have had to cut one or two out before & never try to get between the arm & frame as you will damage both, I prefer to cut the arm as its easier to replace (gives you the excuse to buy something posh too

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If you do go down that route try to cut the arm just past the bearing seat as the bearings are hard