Have you tried the DISKPART menu in the Command prompt in Windows? If not, please try these commands:
1. Open the Command Prompt
2. Press DISKPART
3. Then type LIST DISK
4. When you see a list of records, you should select which disk to be cleaned carefully, because you could delete the information from another one. You can orient yourself on the capacity of the USB flash unit. If you have other drives, the flash may be number 3, for example. If you have other drives, then the local hard disk should be disk 0 disk and USB card 1. Then, write SELECT DISK 1
5. If you put the message "Disk 1 is now the selected disk", write CLEAN This will remove all information and zeros will erase your USB.
6. Then format the flash drive in FAT32 format using the command FS=FAT32
You could also use the Linux Live CD to try to access the USB to delete it.
Let me know if that helped. Cheers.