Those dork discs can be a pain. The easiest way to remove it cleanly is to pop the rear wheel off, then take off the cassette using a chain whip and cassette lockring tool. Once the cassette's off, the disc should slide right off. If you don’t have the tools or don’t want to mess with it, you can carefully cut it off with side cutters or a utility knife; just go slow so you don’t scratch the spokes or hub. But if you want the cleanest result, removing the cassette is the way to go.