[thstone]

Use WD-40, PB Blaster, or similar and let it sit for a day.

Then try again. Use a long handle on the hex socket to get good, solid leverage.

If still no joy, I'd throw a little heat onto it with a propane torch. Of course, you need to be careful, but a little localized heating (not glowing red!) of the plug shouldn't be too risky. its amazing what a little heat will do to loosen up a fastener.