There's a trick to getting these off easily.
When (often) it's not easy, it is because the screw and nut bind together and if you don't hold the nut, turning the screw turns both.

And rather than fumble trying to hold the pedal, screwdriver, and spanner with only two hands, set the screwdriver point upwards in the vice, put the screw head slot on that, and loosen the nut - after that all you should need is your fingers.