If not already done, then create a normal user account (non-administrator) for that user in Windows 7 (via Control Panel->Manage Users->Create new user). This is needed because normal users cannot access/change registry settings. Then login via an administrator account and follow these steps:
1. Go to Start->Run, and type "regedit" in the search bar.
2. The Windows 7 Registry Editor will open up. Go to HKEY_LOCAL_MACHINE -> SYSTEM -> CurrentControlSet - > Control. Right click Control and then select New Key. Add the key name as "StorageDevicePolicies".
3. Select and Right click this new "StorageDevicePolicies" key and select New->DWORD value. Add the value name as "WriteProtect" and set its value as 1(users cannot copy files to USB drives). To allow users to copy files, set the value as 0.
Save and log off, then login as normal user account and now you will notice that the user will not be able to copy files, he will get the "Write protected" error.