the best way to cope up with this issue would be to place a hand on your belly as well. you could term it as a nonverbal sign where they are forced to remove their hands as well. the hands are evading space for which they need to place their hand. most likely they are going to move back without even saying a word. one of the best ways to cope up with this situation.
there is no harm in being to the point and telling them does not touch my pregnant belly. after all it is your body. no one has the right to touch if you do not provide 