What am I able to ban? / How does Draft Pick work?

I got myself into a discussion with some friends and was saying, that you can ONLY ban champions that at least one of the enemy owns (meaning if nobody owns f.e. Warwick you can't ban him because he is not even in the roster). I know that this rule was set up when I started playing League of legends (Season 3) but I believe it hasn't changed yet. Does anyone know further informations?

Thanks for your answers! [sg-ezreal]