First, check to ensure that the GPO actually applies to the computers you want to restrict. Make sure the GPO is applied to the correct OU in Active Directory and that it's not restricted by user or group. Chances are if you just applied to the Default Domain Policy or just created a fresh GPO, the permissions won't be an issue, but it can't hurt to check :). If it didn't work you may need to refresh the GPO. At the command prompt on the client:
gpupdate /force
Restart and log in and see if it worked. If it still didn't work, you could be running into an issue where existing devices are allowed because they were already installed - so give it a try with a device you haven't installed before and see if it is blocked from being installed.
While Charnley's suggestion is a good one, you could run into trouble with that if you ever wanted to, for example, disable USB and CD for standard users but allow it for Administrators (a very simple procedure with Group Policy).
It is, however, a very very very good idea to set a BIOS password and disable USB Boot and Boot from CD, to keep users from booting the machine off a flash drive or CD, into Linux for example, and accessing the hard drive and copying files like that...