Choosing to not use your ban is a perfectly reasonable choice, it is your ban after all, and if you don't mind playing against any champion in the game then you have every right not to ban any of them.
Banning teammates' champions is more tricky. If you're doing it to illicit a negative response from them, then it is absolutely be considered trolling and punishable. But if they do it without discussing it is more grey-area, and might fall under refusing to communicate.
And if they did it with a reasonable explanation then it is definitely not punishable. Example of a reasonable explanation, "You're late in the pick order and that champ is OP I don't want to play against it when the enemy team inevitably would have picked it before you could."