OK thanks, we've just been advised by a consultant that we should prevent the running of executables from removable media. We still need to allow access to files on USB. We'll go the GPO route.
Edit: In case someone else comes looking for this: hxxp://serverfault.com/questions/351535/how-can-i-stop-exes-being-ran-from-removeable-media-such-as-usb-drives