The only way I've learned to combat the advice is to raise polite, well-behaved children. It takes a lot of years to get to that point though, so that's not the best tip lol! Seriously though, it's going to happen regardless - I do notice it more in the baby stage than with the older kids - mostly with the ages they are now, we just get compliments on their behavior and I get asked a lot how we raised such good kids (I'm honestly the first person to say how freakishly good my kids are, because their behavior amazes me), but it's rare that someone tries to tell me what I should or shouldn't be doing now. I'm sure that will start all up again with a baby - I'm planning to combat it by saying we've been doing great with the kids we have, so we'll continue doing what we think is best for them.
As for the belly touching - I hate it! I don't mind if it's family or friends, but the random strangers (usually old ladies) trying to touch it just aggravates me. I actually had an older woman ranting about how she hated when people touch bellies because it invades their personal space and then she touched my (flat/non-pregnant) belly to demonstrate what she was talking about! I was flabbergasted! I said nothing, but a friend was with me and we were both stunned. For the pregnant belly, I tend to back away and cross my arms in a very clear "don't touch" pose. It works some of the time. I'm not a germophobe or anything, but I appreciate my personal space, where strangers are concerned.