I understand the OP's frustration - really getting tired of people thinking that they are so special/important, etc. that the rules don't apply to them. I think, like baggage restrictions, the airlines need to either enforce this requirement or drop it.
On one of my last flights, the girl next to me kept texting after the FA told her to shut her phone down. I turned to her and asked her to please turn her phone off and well.. LOL you can imagine the response I got!!!
No easy answers for this one.....