after executing a back up cmd (via query) how to kill it
There are many ways to kill the backup - exactly the same as you would any query.


ALT+Break in SSMS
Right-click "kill process" from the activity monitor
CTRL+C in sqlcmd 
and so on...(these seemed most appropriate given the implied knowledge of the asker)


It probably won't end immediately but just give it a little time. Also I'd say give it more time in general, a backup of a large DB can take some time and may not start producing stat updates immediately. Check out previous backup times as a guide if possible.