You can stop a backup session that is in progress by clicking Abort on the Backups toolbar.
In the navigation pane, click Backups. The servers are listed in the content pane. Select the server that you want to stop the backup.
Click Abort. This server shows the latest backup session and the status at the time. The backup is immediately stopped and the session status is marked as Competed with Errors.
To see that the session is aborted, click Refresh.