Where can i write a ticket of a suspected boosting/shared account?

Earlier today, i met this Yasuo player who is clearly very good at his mechanics till the point he's being very cocky thus led him to spam GGEZ and EZPZ entire game. Then after checking his history matches, i come to suspect that account is being boosted/shared cos all his previous matches and skill level does not add up. Even if its a smurfed account still does not explain all the previous matches with poor KDA.