I usually just grab the pad with my fingers, and carefully apply torque in the opposite direction of the torque I apply to the screw. The two torques must be roughly equal (very roughly), if you apply too much torque to the pad, you are going to turn it the opposite direction.
It is not enough to grab the pad and try to hold it in place, you have to apply torque proactively to stop the first bit of movement before it happens.
Done right, you can tighten those screws very securely without rotating the pad.
Of course, you need to position the pads properly first, which is easiest to do by applying the brake, adjusting the position of the pads with the brake applied, and fixing their position by tightening the screws with your fingers. You don't really need any torque at that point, as you are just fixing the position. It's when you take out the wrench to tighten the screw for good that you need to apply the counter-torque to the pad with your other hand.
But, of course, a hexagonal surface to grab the pad with a wrench would be nice...