Ratool is a small utility which can help limit access to USB drives.
If you don't want users to be able to copy your files to a USB key, then launch Ratool, select "Allow Read Only", and click "Apply Changes". Any USB drives you currently have connected will still work as before, but if you remove and reconnect them - or try another drive - then you'll find it's no longer possible to copy files there. Attempt to do so in Explorer, say, and you'll be told the drive is "write-protected".
Or, if you don't want people to be able to use USB drives at all - maybe useful if you want to avoid the risk of infection by an autorun virus - then you can launch Ratool, select "Disable USB Disks Detection" and click "Apply Changes". Again, currently connected USB drives will continue to work as before, but plug in anything else and it won't appear in Explorer, or be accessible to your applications.
If this is all sounding a little familiar, then you're right, there are plenty of tools around which do more or less the same thing. But, these tend to work by simply applying a few Registry tweaks, and so it's not difficult to reverse their effects. (Running a similar tool, or just knowing where to look in REGEDIT will probably be enough.)
Ratool, meanwhile, will by default "lock" its changes, making them much more difficult to remove. We tried this, and sure enough, once Ratool had set our USB access preferences, three similar tools weren't able to change them back.
And if you really want to be secure, there's even an option to password-protect the program, preventing other users of your system from launching Ratool and restoring their USB access.