before you go with vice grips or chisiling the thing off do the following,
soak it in liquid wrench/ WD-40 over night. Use the right size socket, whatever proper mm it is I cant remember probablly 12mm. use as few of extensions as possible (only 1 if you can) and if you have the right size socket in a 1/2 inch drive thats better. If you have or your dad? has an impact wrench for an air comressor use that and it will come right off, if not then the longer the handle you have for the ratchet the better, or slip over the handle a strong peice of pipe for leverage. If that doesnt break the nut off use a quick stomp on the end of the rachet while holding the head and the shaft of the ratchet as stongly as you can, works for me every time
Get a tap and Die set t repair thoes threads if that is what is rally going on.