So last Friday my c.v axle snapped in half. So I put it on a trailer and took it home. But as I started to work on it I realized that when I went to take the spindle nut off that since it broke in half it just spins freely when I turn the nut. I can't figure out how to take it off. I thought about maybe trying to grind the nut off?

 

Vise-grips and an impact would get it off. You have to place the vise-grips were it will stop the spinning. Or get a second person to hold the grips as you tap the impact

 

Use your brake to stop the hub from spinning.

 

I'd use a large screwdriver stuck into a ventilation hole in the rotor. Screwdriver comes up against the caliper bracket & stops everything from spinning a lot easier than trying to hold a vise-grips. Same thing if the axle is NOT broken - you don't want the axle-nut torque going into the transmission anyway.

 

X2 on using the brakes to hold it.
I've had a helper hold the brakes on and used a breaker bar when impact tools were not available.
You might also remount the tire without hub cap and lower to the ground then use brake to hold the wheel and use a breaker bar through the center of the wheel.
I've done that too.

Good luck!