How do I prevent standard users from using the USB ports?

You can disable USB storage device in Ubuntu, where only root users can only enable it. Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below.

gksudo gedit /etc/modprobe.d/blacklist.conf

When it opens add blacklist usb_storage as the last line, then with the command below open the rc.local file

gksudo gedit /etc/rc.local

When it opens, add modprobe -r usb_storage before the line exit 0

Now go ahead and reboot your system.

To mount, open a Terminal and use the following command.

sudo modprobe usb_storage

To Unmount the USB device, just right click on the device in file manager, and choose safley remove, or Unmount device, and run

sudo rmmod usb_storage

In Terminal.

^{Source: How to Disable and Enable USB Device}