I agree with Joe to a certain extent, but as a hitter, I have to tell you that if the catcher is setting up the location at the very last moment (which she should be), it would more distracting than helpful to focus my attention away from the pitcher's hip and onto some signal from the coach or the runner at second. Besides, at what level can you trust that the pitcher will actually hit the mit? Can you also trust that the catcher won't set up outside and the pitcher won't throw inside and drill you when you start diving over the plate?
As for signs, if they're stealing your changeup sign, just change it.
If they're making a big, blatant deal out of calling pitches and stealing signs--- like when I saw and heard some MOTHERS IN THE BLEACHERS announcing locations last summer--- then, well, it has to be handled in the time-honored manner. The problem, of course, is that any coaches, players or parents who are ignorant enough to do those sort of things probably wouldn't understand the message when it's delivered. LOL.