As mentioned, remove the scope and remove the barrel from the stock and the trigger assembly from the barrel.

Use the search utility and try some of the techniques mentioned. Also, penetrating oil for several days to a week might work if all else fails.

If you get the plug out, take a very close look at both the plug and barrel threads. If they are galled or damaged, then pitch the barrel and start over with a new one and better maintenance practices.

Or, contact CVA and arrange to send them the barrel for plug removal and inspection. A rusted in plug may have comprimised the threads and the over all integrity of the barrel when it's finally removed. They will be able to remove it and advise if the barrel is junk or not.