When a backup is still indicated to run but isn't really (e.g. after a server crash or media failure), you can try to stop the backup by running the stop command using wbadmin (from an elevated command prompt):
wbadmin stop job
However, this sometimes does not work and the command hangs indefinitely. In this case, you can still stop the backup by stopping the Block Level Backup Engine Service service. If the service does not stop, use Task Manager to kill the process wbengine.exe, which does the same thing.