If they are old enough, a reminder to wash hands afterward might be enough to make them think twice. My oldest did this, and I would remind her gently not to do this in public (she was always compliant). I was more lax at home (ugh, cringing along with you) but the older she got, the easier the gentle message was to get across. I tried not to be judgmental about it, I just addressed issues of hygiene and politeness.
Yup. Time to step away and stop thinking about it. Even when I try to think rationally about it (it's really not that big of a deal, I try to convince myself) I can't quite get past the nose curling and mild shuddering. Logic: for himself, there is no *physical reason* a person shouldn't do this. It is a matter of consideration of others, both in cleanliness and good manners.