USB Flash Drive is not formatting

Hi! Experts,
I can't format my USB flash drive, as it contains some viruses. I have installed a fully functional Antivirus of Avast, but whenever I try to clean it, the Antivirus shows that there are no viruses. But the USB Flash drive has became unstable to use, nothing is going to copy on it nor is it formatting. I'm so depressed about it. What should I do?
Thanks.
