Unfortunately there is no "shake off" the sign button.
All you can do is pick another pitch type besides the recommended one. There are no penalties for picking another pitch type. Just like there is no bonus for following the catcher's recommendation.
Really the suggestions from the catcher are just there as a tool to help you if you dont plan your own sequences. They are merely a recommendation and have no impact on the batter or pitcher.
(I suppose you could shake your head and pretend you shook the catcher off before selecting a different pitch.)