Help with front strut

I'm changing the bellows on both front struts. I have them off the vehicle and on the bench with a spring compressor on them. The next step is to take off the top nut holding the mounting plate on, but when I turn this the whole on the damper shaft turn too. Any ideas on how I get this nut off so I can break down the strut? thanks

i've used an impact gun, all you do is pull the trigger many times in short burst's. but make sure the spring is compressed.hope that helps