If you want to disallow your staff from using external USB based drives, then you need to make some system settings changes in order to make sure your staff cannot access to the USB ports. This even works when you are at a location with USB port but with no access to it, you just need to make some registry changes in your PC.

How to Enable USB Storage Device Access :
If you already have a USB port on the PC but without the access to use it then you need to perform the following changes.

1. Click Start, and then click Run.
2. In the Open box, type regedit, and then click OK.
3. Locate and then click the following registry key:
HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesUsbStor
4. In the details pane, double-click Start.
5. In the Value data box, type 4, click Hexadecimal (if it is not already selected), and then click OK.
6. Exit Registry Editor.

How to Disable USB Storage Device Access:
In order to keep your PC secure you can disable total access to the USB Storage devices.

1. Click Start, and then click Run.
2. In the Open box, type regedit, and then click OK.
3. Locate, and then click the following registry key:
HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesUsbStor
4. In the right pane, double-click Start.
5. In the Value data box, type 3, click Hexadecimal (if it is not already selected), and then click OK.
6. Quit Registry Editor.

Generally its not recommended to allow access to USB Ports at offices and public PC unless required because the computers can get attacked by virus very quickly. [Amazing USB Drives]

Note : Make sure you backup your registry before making any changes to it. You can easily restore it if you feel you have made any mistake in making the registry changes.