Programmable remotes

[David Etra]

We have a private car park protected by a gate and our users can open the gate using hand held remotes (we are currently using the Jane ones). There are 24 parking spaces and we have about 50 users, but we sometime have a problem whereby users abscond taking their remotes with them and then use the car park when they should not. The radio receiver and control board for the gate are obsolete and starting to give us trouble: we need to replace them. We would like to replace them with a system whereby we can authorise the remotes individually and then de-authorise them when any of our users leaves the scheme. We have looked at GSM systems but don't like the idea of users being able to open the gate when they are not within sight of it, so we would rather stay with the 433MHz system (or perhaps Bluetooth?).
Are there any systems available which would allow us to authorise remotes on an individual basis?
Thanks - David.