If you don't need it anymore you should be able to format the USB drive by
right clicking it an selecting format which will erase all data on the drive
if you do not need anything on that drive. The operating system will not
recognize it anymore but if you want to destroy that data you need to use a
third party program to scrub the drive as files erased by the operating
system are often recoverable if that is a concern. There is a free program
called Eraser that could take care of that.
Steve
http://www.tolvanen.com/eraser/ --- Eraser