Open CMD : (Start - >Type CMD -> Press Enter)
Type "diskpart" and press Enter.
If it ask allow Access - Press YES.
Now a New CMD wil open,
Type "list disk"
It will list all your connected drive.
Select the drive of your pen drive.
Type "select disk #'
# is the number of your drive.
In this Screenshot i not connected my pen drive, so i selected my Hard disk which is "0"
Type "attributes disk clear readonly"
Done !! - Now your Pen Drive is Formatted !!