How do I remove the nut?

[annastrandh1]

Help to remove the nut?
I can't manage to hold the screw (or whatever it's called) in place so I can remove the nut without the screw spinning along. Tried for ages. There's probably some simple trick.

 

[AFH]

Place two nuts on the short thread at the top of the image. Tighten them against each other, lock them. Then you can unscrew the stuck nut.

 

[Daniel_N]

Tighten two nuts against each other (locknut). Then you have a fixed point and can loosen your nut.
Or hold with, for example, slip-joint pliers on the smooth part (but it can easily leave marks)
Or clamp the threads in a vise with wood in between to protect them.

But a locknut is best in this context.

 

[annastrandh1]

I've had no luck with Polygrip. So I'm going out to the woodshed to look for kontramuttrarr then.

 

[annastrandh1]

Worked fine! Thank you very much!