WININSTALL usb

I have installed Windows 8 via Boot Camp earlier, then I deleted and removed the Windows partition. Though, I originally created the usb which installed windows, called WININSTALL, and I want delete everything inside which originally was used for installation perviously. When I put the usb into my Macbook, it could detect t and open the files, but when I tried to delete those files, it wouldn't let me because they are protected.

My Macbook: Macbook Air 13' inch (Mid 2013) - OS X Yosemite

Any solutions?