How to deal with that?

First of all - sry for bad english. Today i encountered player in soloq, who had a pomor in master tier. After game start, enemy midlaner(wukong) says in chat "Thx for a boost" and just goes under our tower and dies. Our midlaner(GP) still not connected. Then enemy jungler says in chat that their midlaner(wukong) leave a game. After 15-20 seconds, our midlaer(GP) reconnected. In post-game lobby our midlaner siad same thing, like enemy midlaner "Thx for a boost". After that I decided to check his account on, and i found that he "played" a couple of times agains't their midlaner and all games are win, like that. And one thing makes me think that these two accounts belong to one player. Our midlaner. Enemy midlaner. Honestly, this is very obviously that he is wintrading from his second acc to first. And i don't think that system will trigger if i report his first account, he did "nothing bad" on this account. For real, how to deal with that? Well even if riot ban his second account, i think that this will do nothing to him.
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