In general, banning any champion for any reason is allowed, and not punishable at all.
But, when you do it to annoy someone and EXPRESS IN CHAT that you've done it to annoy them, that is as punishable as any other expression of intentionally annoying someone. It will contribute to their toxicity over time score, and will contribute to their next punishment.
Additionally, if you complain about it, or start an argument about it, that will be as punishable as any other expression of complaint or any other starting of an argument.
So, the advice I give: When someone bans your champion; let it go. Assume that they had a valid reason to take that action, even if it was not paying attention. If they did it to annoy you, they will certainly gloat or taunt, and that will open them up to punishment. If you start an argument about it, that will open you up to punishment, and there is the possibility that they banned for a valid reason.