2007 Peugeot 207 rear caliper piston

We can try a few things before actually going ahead and replacing the caliper. First, I'm not exactly sure which tool you are using. If you are using a regular hand turn one, you may not be getting sufficient force to push while turning at the same time. There are some kits out there that have a screw type setup which is usually the better option. The adapter sits in the grooves of the piston, then there's a plate that sits up inside of the caliper while the screw end sticks out of the caliper. This screw end should have an end to be able to use a wrench or socket to spin it inwards. Before any of this, I like to take the cap off of the master cylinder. I do this because the cap actually holds back the pressure and actually fights against you. If this doesn't work, try cracking the bleeder screw to relieve the pressure and try to compress the piston at the same time. Be sure to have enough fluid in the master cylinder to prevent air from being sucked in. If none of these work, then you may have a defect with the brake caliper and I would go ahead and replace it at this time.

Ok so you knew exactly which tool I was talking about. That unfortunate that it didn't work. Let me know what you find after relieving the pressure by cracking loose the brake bleeder.