The champion that you pick cannot be banned. However, the one you declare or want to play can be. And that's what it is - it is a request to play that champion, a request that, within the rules of the game, can rightfully be denied by the one doing the bans.
But if you add in the chat something along the lines of "let me have [champion], please", making it a formal request, then, depending on the reply, it may be considered negative attitude at best if someone still bans your champion.
If someone still bans it and posting ";)" in the chat, I'd consider that to be harrassment. A "no" without reason and bans it, negative attitude. No reponse and bans it, negative attitude.
If "no", but with a subjectively good reason included, I'd consider it a valid reason to ban the champion.
This is also why you should have at least a handful of champions you know how to play. So when something like this does happen, you can play something else and do just as well. Holding resentment over someone banning your main does no one any good, as you should be prepared to will happen sometimes.
If someone flatly tells you what to play for whatever reason, that is harrassment. If they argue what the team needs, and you say you don't know how to play that class, that is their problem and they are free to dodge if they want if the issue can't be solved with you playing something else.
