I'd probably pour some breakfree down the barrel or wd-40 etc. Let it sit overnight, so there's time for some to work down in the threads. Just shove a piece of rubber or cork into the primer pocket too keep it from drizzling out. The next day, flip it over and spray some breakfree on the other side around the flange of the breechplug. On day 3, get ahold of an impact (not air impact) ratchet, the kind you hammer on and chuck up a socket sized for the plug. Make sure it's the right size, then give it a couple wacks with that. Should loosen and free up after that.

If it doesn't, give it a couple days each side maybe for deeper penetration.

***edit***
If you don't have a hex style plug, Just put an aluminum or brass rod against it and rap it a couple times with a light hammer. You don't want to peen it or shatter it, then try the wrench again. Sorry I forget not all are hexes.
