What's the best way to remove stick-on frame protectors
It depends on the type of adhesive. Sometimes it can be just pulled off. Sometimes the gentle heat of a hairdryer (not a heat gun) can weaken an adhesive and make it easier to remove. 

Or you can try the use of GooGone or similar adhesive thinner/solvent (try on an inconspicuous place first to make sure it won't damage the finish).

Heat and solvent cover 90% of cases. 

If the protector was epoxied on, then it is very hard to take off without damaging the finish.