John Deere Mower blades

Last year I purchased A John Deere mod. S240.
I built some ramps to raise the mower, so I can change the blades without
removing the deck.
Alias I cannot remove the nuts from spindle shaft.
After blocking the blades with wood the nut will not turn.
I tried WD40 and a breaker bar extension on my socket handle. No luck
These blades were put on at the factory!
Any suggestions?
Thanks