I'd start with a jam fit Torx diver. Use a grinder to make the end flat and square, then with the hanger supported, hammer it ino the hex so it cuts it's own way. If it works, you an probably reuse the bolt. If not, drill it down, or through with a 5-6mm or do drill, and use an easy out to remove it.
Last ditch plan, drill it through with a 10mm or so drill, which will cut it apart a the hanger. Sadly this will mean the loss of the hanger, but if the easy out didn't remove it, the bolt is seized, and threads shot anyway.
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