Person who prohibits placard? (6)
I believe the answer is:
banner
This clue is a double definition.
'person who prohibits' is the first definition.
(someone who ban things)
'placard?' is the second definition.
(a placard is a banner or poster)
(Other definitions for banner that I've seen before include "Large fabric sign" , "Queen brought in British flag" , "Standard carried at demo etc" , "Military standard" , "Knight's standard" .)