A common security concern at organizations is allowing users to plug in a USB flash drive, because they could so easily copy corporate data.
Since Windows XP SP2, you can disable writing to USB devices altogether using a simple registry hack. Here it is:
Once you use the registry hack, you will have to reboot for the changes to take effect. One should also note that if you are using this trick, you should make sure that the users are not administrators on the computer, because they could easily change this setting back.
This works on Windows Vista as well. Here’s the window you’ll get when you try and write to a USB drive.