I have often attempted to cancel backups at various stages and found them to hang in the "cancel pending" state forever. A server reboot tends to put things in order, but is a fairly inconvenient solution. Stopping the services from the Windows Administrative Tools | Services snap-in tends to hang the services in the stopping state as well.
The easy but usually overlooked method is choosing Tools | BackUp Exec Services from within the Backup Exec console. You can stop and restart them all fairly effectively from there.
Stopping may take several minutes, but is ultimately successful.