Say loudly, "Sir, in seat XX, please turn your phone off NOW." If you're speaking in a loud, commanding tone that a lot of other people around you can hear, the shaming factor has great power. I do this when provoked on the Amtrak Quiet Car, and it works like a charm. If the offender has no compunction about endangering his/her fellow passengers (or just breaking the rules in extremely irritating fashion), then you shouldn't feel bad about speaking as if you're disciplining a three-year-old.