thanks for all the advice on removing blades - we've tried the wood, spanner, hammer, wrench - don't have an air or impact wrench - thing is, do these things come off anticlockwise or clockwise? Please don't laugh!
Not a sily question, but what model tractor, and what mower deck. I have 3 decks 2 are clockwise and 1 is counter clockwise. When you say you tried the block of wood, were you able to get it wedged in good enough so it wouldn't slip?
As written, it depends. You might have a rust problem or be forcing the other way. Usually I block the blade with a 2x4 and put a power bar and give it a whack with a rubber hammer. Usually thats all it took. If it didn't move, I'd try the other direction.
On most of the decks I have worked on,if the when viewed from underneath the blade turns clockwise the nut has been LH thread.
Edit: but not if there is a locking tab!
Dave