I remember having this twice when our kid was 1 or 2, each time it was a couple of gossiping a-ma's, although we were travelling both times and it was easier to confront. They’ve got nothing better to do, presumably their own kids (if they had anyway) have grown up, and they feel obliged to comment on the parenting skills of the big-nose and his wife.
On the one plane, this woman was really pissing off my wife, loudly saying that we obviously weren’t feeding our daughter properly, and we should be doing this, shouldn’t be doing that, on and on. In the end, I went over and politely told her to STFU. She did (or at least I couldn’t hear her after that).
It’s trickier if it’s around home, but I think you still got to confront him/her. Tell them to mind their own business, don’t mince your words. Impose yourself on the neighbourhood. Have the wife and child there as back-up.
Wouldn’t even consider legal stuff right now. This should be in Parenting forum. Just throw your weight around.