You can always try sending a report ticket. I'm not sure if intentionally banning someone that another player intended to play is against the rules as you can easily say you weren't paying attention or had to do something last minute and then came back and quickly banned without seeing someone else intend to pick x champ. So it's a bit of a touchy subject similar to feeders how you can't always tell if they intended to feed or not. But one thing I tend to do personally is never show which champ I intend to play because I know the potential of a troll teammate who will intentionally ban my intended champ. Usually happens to me especially because if I get a role of adc and I intend to pick nidalee, since it's unusual to have nidalee as adc, people on my team will intentionally ban nidalee so I can't use nid as adc. Or say someone doesn't like taric support and I wanted to play taric support, so my adc intentionally bans taric. That's why I just never show my intended champ. But then this is irrelevant considering I don't really ever play draft or ranked anymore.