Sometimes what is not politically correct is the right course of action.
I can relate to you. Our daughter (age 3...that was 9 years ago) kept pinching and just wouldn't stop. Nothing we tried would work.
One day, in the grocery store, I warned her (gently) that if she pinched me again I would pinch her back. She proceeded to pinch me. I VERY GENTLY pinched her on her arm (not enough to hurt) and she screamed out, "Mommy PUNCHED me!" She kept yelling out that I had punched her! And with every turn of the aisle, I kept waiting to be arrested by child protective services! Boy, I never did that again! But she NEVER EVER pinched anyone again, so apparently it worked.
The child turned out beautifully...an honors student, an altar server, a signed model and a team athlete in both tennis and soccer! I'm quite sure your child turned out great, too!