You can't stop the installation of DTM through Policy, just limit it's uses. Even then they have to EA first and then cradle via the USB for it to apply to the DTM software on the Desktop.
Sounds like they have to connect with USB at some point, unless you outlaw the Desktop software
You could allow a basic install of just the Device Manager. It just installs the drivers so it can charge faster. There is no gui installed so they can't use Application Loader or anything for that matter, just charge it if the battery is low.