Chastises (7)
'chastises' is the definition.
('castigate' can be a synonym of 'chastize')
This is all the clue.
(Another definition for castigates that I've seen is " Slates".)
(Other definitions for rebukes that I've seen before include "Admonishes sharply or severely" , "Admonishes, chides" , "Expresses sharp disapproval" , "Strong reprimands" .)