I can't remember the size of the large nut at the top. 22mm possibly?

But you could fit a socket over the top of it and grip it with a large pair of mole grips, then turn the 7mm allen headed thread with an allen key.
I have the correct tool for the job but I don't live near you.
If you are going to bin the strut, then you can drill the allen headed thread part out of the nut and remove it that way. Butchery, yes, but it does the job and you will need a new nut. The threaded part is very soft metal, so it's easy to drill and also easy to round off if the allen key you use has passed it's sell by date.
