I think you have omitted some vital information here. Have you actually installed MInt from the USB stick or are you running it from the USB stick all the time? If you are running from the USB stick then you are not going to be able to use it for anything else. If you have installed Mint to your hard drive you should be able to format the USB stick with gparted.
Run gparted, change the drive to the USB stick (drop down box, top right of gui), right click the partition/s on the stick and select 'Format to' then choose what filesystem you want, click apply a couple of times and the job is done.
Fujitsu Lifebook AH532. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Lubuntu 13.10,Ubuntu12.10 (Unity), Mint16 (Cinnamon), Manjaro (Xfce).