Just got a booster in my game boosting an account that is not his - How can I report him?

Just got a booster in my game, I dont wanna post the name here but just look at my match history, I played as annie, and the graves in my team is getting boosted right now. That was his first game being boosted, thats not his account nor his elo, just look at his match history all losses, then all of a sudden played like a high elo in my game. How can I report boosted acounts?? Thanks
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