I did this job without a spring compressor a few years ago too, but in a slightly different way to that described above. I would never trust a hydraulic trolley jack to be supporting the arm all the time, so I jacked it up high enough initially, then rested the arm on wide solid wooden/concrete blocks. The other wheels were also chocked.
I then cut a piece of 1/2" ply to form a shield between me and the spring. I also tied the spring to a few lengths of wood so if the arm dropped, or car moved and the spring ever became free, its path to freedom would be severely hindered. (this was very unlikely anyway, as the spring would have to lift the car itself).

Then loosen the strut nut, and jack up the car from the front crossmember up to remove it. Lower the car then onto an axle stand at the front jacking point. Now the wishbone can be levered, pressed, tightened etc with the car and arms on solid surfaces, not a jack. Might be a bit long winded.....but I had heard of a local man who was removing springs and killed himself. I didn't want that to happen, so I dont mind putting in a few extra belt and braces.

The risk of the trolley jack failing, or slipping off that sprung control arm is very real.