Why doesn't Riot ban boosted accounts?

I have reported the same boosted account 4 times, nothing has been done yet. It was painfully obvious aswell. A hard stuck plat 4 Zed main magically picks up Yi and goes on a 20 game winstreak while also using the Chinese language in those 20 games. After the boost he hasn't won a game and has lost 7 in a row. Why does riot just disregard this huge problem with the game?
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