After many such instances, I do not use zipper boots anymore.
To get them free, I used a pair of pliers to grip the tabs and pulled like hell. Eventually one of the tabs broke off. That was the last pair of zippered flats boots I've ever (or will ever) own.
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
and Founder, Worldwide Flyfishing Forum