Pull a “Urgh that’s disgusting” facial expression at them. Best if done as if it’s just a joke. I’m yet to find a successful method, but find this ones the best I’ve thought of to date.
My gf is talking on the phone to her friend and eating a takeaway at the sane time. I’ve told her hundreds of time how disgusting it makes me feel, yet she still does it. Just can’t understand people doing it. It’s revolting.
Typing on here is a bit of therapy at least!