The passive form was used because someone else banned using it . That`s why we use these sentences. ** Smoking is banned in some public buildings. (e.i : it is banned by the government) **Smoking and mobile phones are banned on flights. (e.i : like the above one) I think hunting should be banned. (i.e : should be banned by the authority )

Smoking is banned in some public buildings. > Smoking is singular , so use 'is" Smoking and mobile phones are banned on flights.> Smoking and mobile phones are plural , so use 'are' I think hunting should be banned.> should *to become* banned . The past particle of "ban " is banned . But, some past particles can be used as an adjectives > the banned book , the torn book, the coloured book , the ruined book Smoking is banned in some public buildings .> banned here is used as an adjective > meaning in a state of being banned. What is banned ? Smoking ? Who is beautiful ? She is . Or,the book is ruined > in a state of ruin. Banned as a verb : I have banned him/I am banning him