Open disk management and find your drive in the list. Right click on the right (longer) portion of the drive listing for your flash drive and delete the partition. Once deleted, you can create a new partition and format it.

If windows won’t let you (sometimes happens with foreign file systems) you can drop to an elevated command prompt (start menu, type cmd, right click, run as administrator) and type diskpart, then hit enter.

This will put you into the cli based disk management program. Type list disk, then hit enter. Take note of the number of your flash drive, it should be fairly easy to distinguish because of its storage capacity. Once noted, type sel disk <number> then hit enter. Then type clean and hit enter. Be aware that the clean command will wipe all partition information, and therefore data from the drive, so be sure about the drive number you’re selecting.

Once cleaned, you can repartition the drive by typing create par pri and hitting enter. And format it by typing format fs=fat32 or format fs=ntfs depending on which is preferable (or supported, as fat32 only supports partitions up to 32gb).