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