Use a USB-condom. This is a device that plugs in between your normal cable and the computer and blocks the data lines

Use a USB cable with a "data switch". This cable is normally power-only, which is what you want 90% of the time. However there is a button ("Data Transfer Protection On/Off Switch") you can press that will enable data. An LED indicates the mode. This kind of cable is much safer and secure, plus more convenient for the users. It follows the security principle that if you make the defaults what you want users to do, they're more likely to follow your security policy.