You can write protect a USB (or any drive for that matter) using DISKPART.

With the drive plugged in open up a command prompt and enter DISKPART.

Then list the disks in your system to determine which one is the USB drive using LIST DISKS

Once identified, enter SELECT DISK #, where # is the number corresponding to your USB disk.

To list disk attributes enter ATTRIBUTES DISK.

To set disk to read only enter ATTRIBUTES DISK SET READONLY.

To remove read only restriction enter ATTRIBUTES DISK CLEAR READONLY.