it is possible to disable the original game radios to let only those created by me?
Yes!
I think you cannot disable them, but just redirecting to your radio when a player wants to hear a default radio.
There is a Vehicle.RadioLocked property. You could set the radio station with Vehicle.Radio and then lock it.
Can't really think of any other way. Other then checking the radio station every few seconds and set it to your custom one if it's a default station.
Custom radios can't overwrite default ones because they use IDs higher then the default radio stations.