Since you will still need the USB ports for printers/scanners/etc you're going to need software to do the job.
The two main players in this field are Securewave and Pointsec. Both offer excellent products, and I suggest speaking to the companies, or downloading a trial, to see which would suit you best.
In the end, we went for Pointsec’s offering, a product called DisknetPro. We found it far more granular and configurable, and has a rather useful facility for those people who are authorized to use USB keys to perhaps take stuff home.
DiskNet will encrypt the USB key, and will also place a checksum file on it. If the user takes it home, alters anything on it, and brings it back, as soon as they plug it into to a company PC, disknet will detect that the checksum has changed and will log this, and can be configured to notify the IT dept to allow access once more. It will also scan the device for viruses using your own AV and also for malicious programs.
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Matt
Life is all shadows and dust.
Live it up with women and wine while you can