YOU ARE IN A PANEL OF INTERVIEWERS AND YOU NOTICE AN APPLICANT IS NOT QUALIFIED,WOULD YOU INFORM HIM/HER OR WAIT UNTIL THE INTERVIEW WAS OVER.

First you should blame yourself for:

- not going through all resume;

- wasting candidate time;

- wasting your time.

If we assume that was a recruitment agency event .. definetely you need to change the partener. 

However, professionally speaking, the interview can be over any time you, or the recruitment team,  consider appropriate to be over. Also, professionaly ethics should prevail and you should tell the candidate the thruth and eventually advise the direction should go.

I will wait till the completion of his / her interview and guide him / her properly that how he / she can polish himself / herself.