You will find it is an offence for the driver or any passenger to not wear a seatbelt in any country in western Europe. On top of that the driver gets the fine for each passenger not wearing a belt.
Not only that, I doubt you'll find a company that will deactivate the chime as I believe it's an offence to tamper with safety systems regarding seatbelts.
Furthermore, if these systems are found to be tampered with then your MOT and insurance is void.
I just can't understand why passengers wouldnt want to wear a seatbelt, the world as moved on since the 1950s/60s & 70s.
I'd advise your passengers to watch videos on what happens when they don't wear a seatbelt. Needless to say, the one sat behind you can ultimately kill you as they hit the back of your seat!