It's just my opinion but I believe that if a piece of verdigris is flicked off and there is nothing left on the coin and the coin is not exposed to an environment that encourage the reaction needed for verdigris to occur then it will not come back.
The problem we have is controlling the factors that cause it.
If we think about the above example of a coin with a spot of verdigris that is loose on the surface, at some point the conditions were perfect for it to start appearing maybe it was a small piece of sweat from a finger or moisture from a breath. Whatever it was it landed on the coin and started the reaction with the metal, maybe it was stored in a room with fluctuations in temperature and a spot formed.
Then someone comes along and flicks it off, as long as it loose and comes away clean and completely then that verdigris is gone.
If verdigris reappears on the coin then the conditions for it to be there have not been controlled e.g. Poor handling, storage, temperature etc.