Well then don't give them money to do so, its not like phones grow on trees. If you make selling phone/internet device to a minor under certain threshold an illegal act severely punished by law in same way alcohol and cigarettes are, many cases of access are solved. Also, paid internet subscription doesn't grow on the trees even though there are free wifi networks.

All imperfect solutions, but they slice original huge problem into much smaller chunks which are easier to tackle with next approach.