I mean, I have accidentally banned someone's champion without realizing it. I will be looking at the team during the entire pick declare phase, so it's rare for me to accidentally ban someone on my team's champion, but it has happened, specifically when I'm first ban and someone declares after the initial pick declare phase. I've also had my champion banned on accident, and it's alright because it's a mistake. It certainly shouldn't be bannable.
However, I have also had someone on my team declare that they're taking my role, then someone on the team picking/banning my champion and then, if it wasn't the person who declared they would take the role, swapping with that person later. This kind of thing, taking/banning someone's champion intentionally for the purpose of stealing the role or stopping them from playing a champion is bad and is bannable.