Rear Passenger Strut Loose and Spins

I replaced the struts and springs in my 1991 Miata. The rear passenger side strut seems to have a loose nut in the center. It shifts up and down and makes a clunk noise while I drive. The problem is, when I try and tighten it the whole strut rod moves with it. I tried putting a rag and then a hose on the rod and used vise grips while tightening but it just continues to move. Do I need to take it off and compress the springs? Maybe my top mount is bad? I'm lost and would really appreciate the help!

EDIT- ended up replacing the top mounts and that let me tighten it up enough without it spinning.