For me, you shouldn't ban an ally's pick unless:
- It's a newly released champion in ranked - GTFO go to normals for new champs
- It's ridiculously OP and they're a late pick. E.g. pre hotfix Sona
While there are other things that could be seen as reasons (e.g. the team comp is awful and that champion is the primary reason - e.g. a Yasuo/Zed/Talon mid in to an AD top, ADC, AD assassin jungler, and non-high-magic damage support) but I don't really support these as they'll likely make the player rage and be worse than just letting them pick.
Although on that note - people who go AD mid in to all AD team comps without using the pick intent feature need to smarten up. I'm willing to go Corki ADC, I practiced him explicitly for those team comps. I'm willing to take a different top laner. I'm willing to swap to mid lane for you and take some magic damage champion. So why are you forcing an all AD team comp with no notice...