Unscrew the bleeder screw to relieve any pressure on it and then try (you'll have to bleed it). And have you tried turning it in like a bolt. I know a lot of rear caliper pistons are threaded and you have to turn them to get it back in. If it's not the threaded type, just keep working at it with the C-clamp and it should go. Keep moving the clamp to different spots on the piston (top, bottom, left, right), at first you might not think you're moving it at all because you can barely see it, but it's moving, it might take a few minutes of doing this, but there's no reason that it shouldn't go in if the pressure is relieved.