The poster idea would certainly be the cheapest option.
For those that think using a mobile before pushback is ok, consider this -
On a short turnround the self loading cargo may be on board whilst the aircraft is being refuelled. It is theoretically possible for a mobile phone to cause a spark when transmitting or recieving and with that much fuel around does anyone want to test the theory!!
The only guaranteed way to ensure all phones are OFF is to remove them from the hands of untrustworthy pax before boarding and hand them back after the flight.