If your backup job is waiting for a new tape, sometimes it won't automatically cancel after the time defined in the selection list properties.

If you only use a single tape per day and don't want to span, you can configure BE to automatically cancel the job when it receives the Media Insert alert.

1. Go to the Alert Tab.  Then on the left click on Configure Alert Categories.
2. Find Media Insert (I assume that is what the alert you are getting is).
3. Check Automatically Clear After ## hours or minutes, with Cancel.
4. This will Cancel the job, but have whatever you did get on to one tape.

See also: How do I end a scheduled task without rebooting the server?