I need to remove a nut holding on pulley on a mower deck. I cannot hold the pulley stationary while trying to remove a stubborn nut. I cannot take of the nut with the pulley spinning. Is there a tool that holds the pulley stationary while unwinding the nut?

BB in Baltimore

An impact wrench is the easiest way to remove the nut.

But, if you don't have access to an impact wrench, you can very carefully (the blades can be very sharp) hold the blade under the deck while you are trying to take the nut loose. You will get much more leverage this way.

I think the nut is attached to a bolt that goes through the pulley but it is not attached to the pulley. The pulley is turning on you but there should be a nut on the other side of the housing. Find that nut and put a socket on it and then turn your bolt head. The pulley spins loose in all situations.