The cause of your problem is probably your failure to use the /interactive switch with the AT command and NTBACKUP is trying to issue a message.
In tip 656, I described how to kill a process using KILL from the Resource Kit.
If you don't have the Resource Kit, you can:
AT HH:MM /interactive taskmgr
where HH:MM is two minutes from now. When Taskmgr starts, it will do so under the same account context as the scheduled NTBACKUP.
It should then be able to kill the process.