The answer lies in local variables, that are only recognized by one computer in a MP game. My favourite is the variable "player". Assuming you want to hide a Radio Alpha trigger from a unit named nominesine, but not from a unit named Ryote you can write:
?nominesine == player: 1 setRadioMsg "NULL"
in the init.sqs
Init.sqs is run on all computers, clients as well as server, at mission start. Since "player" will return a different value on all computers the condition will be true only on the computer where nominesine is the local player.
On the computer were Ryote plays, the condition will return the value false (because on that computer Ryote == player)
Ergo: nominesine will not be able to use the Radio Alpha trigger. Ryote will be able to use it. Good luck with your mission.