(Closed browser while writing reply, it deleted my explanation of tagged strings)
Messages like that don't exactly work in that way. When a message from the server which comes with an identifier (the first argument of messageAll(), etc) it is either sent to the chat if it is unrecognised or it calls a special function which handles the message and information. msgPlantError_overlap doesn't play base/data/sound/msgPlantError_overlap, it will show the Brick Inside Brick error.
I'd use clientcmdAdminSuccess() (commandtoclient(%newAdmin,'adminsuccess')), which plays the sound and actually lets the player access the admin menu and load bricks menus.
You can't disable the admin menu for a client, but the functions are useless as the buttons do nothing when you are not an admin on a server.