You can disable Mini-USB ports on the SRX Series Services Gateway to block users from connecting a USB mass storage device to the services gateway or services router. When you disables the device, any transactions in progress on the USB device are aborted.
set chassis usb storage disable
Enabling the same back:-
delete chassis usb storage disable
Checking the status:-
show chassis usb storage
Note:- Juniper Techpub Documentation for the same is under process.