Strangers touching your pregnant belly!!

Ok- let me start by telling you that I always had issues with unsolicited touching and strangers standing too close. Even hugs feel kind of weird sometimes... BUT now that I'm pregnant people think it's ok to touch my belly! (It-is-not!!). It's still early and I don't show much yet, so so far only people I know have done it (I jumped back in horror). But I cringe just to think of my reaction when a stranger does it! I saw it happen with my best friend - all the time. What to do?