To make a bootable usb unbootable using command prompt

1. Open Command prompt
2. Type diskpart and press enter
3. Type List disk and press enter
4. Type select disk N(N is your disk number starts from 0) and press enter
5. Type clean and press enter
6. Type create partition primary and press enter
7. Type Active and press enter
8. Type format fs=fat32 quick and press enter(wait for few seconds until the format process reaches 100 percent)
9. Type Assign and press enter
10. Type exit and press enter

This is one of the method to unboot the bootble USB using cmd. Hope it will help you.