How to report a boosted account?

I'd like to know where to report an account that has obviously been boosted. A person was bronze for the last 3 seasons now he's suddenly pulling a 90% winrate at gold MMR and has like 20 kills every game. For example,now he has a 95% winrate with Riven and an average KDA of 7.19.
Report as:
Offensive Spam Harassment Incorrect Board