The only way to remove a really stuck piston is to use the hydraulic system on the vehicle. Get a C-clamp and securely clamp the functioning piston all the way back into its bore. Make sure the C-clamp won't interfere with the stuck piston when it moves away from its cylinder.

Connect up the brake hose and support the caliper so that the brake hose doesn't take all the weight of the caliper. If you introduced air into the system in a big way, you may need to bleed the caliper before attempting this procedure.

Get in vehicle and step on the brake pedal several times. The pressure in the system will force the stuck caliper right out of the bore.

Now, you can determine if the bore is in decent condition and the caliper can be salvaged, or whether it needs to be replaced.

I salvaged one of my rear calipers on the SDL with a slightly binding piston. The bore cleaned up fine.