I hovered a champion to play and was confused when my teammate banned them, later in the game I learned he did it out of spite. This kind of thing really gets me angry, especially when I'm used to playing a specific champion recently and someone on my team bans it out.
I would say that's griefing. It would be different if he didn't want the enemy team to take that champ.
just report them for negative attitude, verbal abuse and hate speech, that kind of player def does all those in his games so your reports will be valid
It can be done for the purpose of causing grief. It's possible for the person who's champ was banned to feel grieved.
Riot does not consider it, by itself, griefing. It's considered to be a viable gameplay action, and just a tactical disagreement.
As such, responding poorly to that action can be punishable, based on the reaction. Take care to keep yourself controlled.
From my experience, if a person is taking a specific action to cause grief, they will continue to do so with other actions. Those other actions will probably be considered toxic by Riot, so report them for those other actions. If they don't take any other actions, it's possible, even likely, that their banning decision was based on team composition/pick order/personal preference rather than trying to grief.
