This is a common problem and removing the pads and pushing the piston out with the brake lever will expose enough of the piston for you to clean it up. Clean any obvious dirt or oxidisation off it but try not to scratch the piston. It will difficult to push back the piston back into place but a G clamp or similar can be used. If the piston is badly damaged it will have to be replaced along with the rubber seal. When you have finished check the brakes and spin the wheel to see if is still binding. A tiny amount is of binding is ok as the amount the pads move is so small.