Are you an administrator on the machine?
You can try one of these methods :
Method 1 : Follow the following steps.
- click on start, type "cmd" in the search bar (no quotes)
- right click on the cmd.exe that pops up and choose run as administrator.
- type the following in the command prompt "cd /" and hit enter (no quotes)
- type "format" followed by the drive letter of the pen-drive (eg. "format e:" if E is the drive letter of your pen-drive.
Method 2 : reboot your computer into safe mode, try to format from there
If none of these don't work to get the drive formatted, there might be some other things we could try. However it will be slightly more complicated than these, and I'm fairly certain one of these options should work.
Best of luck.