Here is a simple registry hack to do so, try it with care.
1. Go to Start –> Run, type Regedit.
2. Go to HKEY_LOCAL_MACHINE\System\CurrentControlset\Servic es\USBStor
3. In the right pane, look for value Start and have value as 0000000(3)
4. Double click on that and change that value to 4.
You might be required to restart the PC, and then you are done with it, USB is now blocked.
In case if you want to get the USB unblocked, just change the value again to 3