If they're false there's no stopping them (5)
I believe the answer is:
teeth
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for teeth that I've seen before include "Incisors, say" , "Dentures" , "Structures in the jaws" , "One may be armed to them" , "Canines, maybe" .)