To be honest, if it was someone I was really close to (like a boyfriend), and i was feeling a little evil that day, I would be tempted to ':
Put two fingers under their chin to keep their mouth shut, lean in really close and stare them in the eye, without blinking, and continue with what I was saying. Then smile (calmly or warmly, not smugly), lean back and continue the conversation as normal.
Every single time they did it.