Try the below mentioned steps and check if you are able to format the pen drive:
1. Shutdown your computer.
2. Insert USB Memory Stick into your computer.
3. Power on the computer and then immediately start tapping (F8) key then "Advanced Boot Options" screen will come up.
4. Select "Safe Mode with Command Prompt".
5. After booting to Safe Mode with Command Prompt, if your USB Memory Stick drive letter is (G) for example, then type:
G: and press enter
6. Then type:
format G: and press enter
7. If it asks you (Y/N) then press (Y) and press enter
8. This will do a full format of the Pen Drive.
Note: All the files in the Pen Drive will be removed after Format, so please backup all the data in the Pen Drive before formatting.
Do reply with results.