Hey, You!
Grab you some pliers, a ratchet wrench and a good helping of WD-40!
Here's your to-do list:
1-PIN down the pliers onto the motor's extruding metal rod thingy (Where the belt goes, the "Spindle", if you will!).
2-SPRAY a sufficient amount of WD-40 penetrating spray into and around the nut. this will penetrate into the nut and help to free it easily.
3-WRENCH the nut off with the ratcheting wrench.
This should VERY easily help with the removal of the nut. if this fails and you have a blow-torch, you can heat the nut up after wiping off the WD-40. This will expand the nut and thus make it more willing to remove.