How to stop multiaccounters?

Every Tuesday and Thursday at my college a player will take all possible gyms there and quickly add his Pokemon from 2 accounts. He often has other legit players with him but they always have the edge due to his multiple accounts. I have reported him multiple times to Niantic but he is still prevalent. It's really making the game not very enjoyable when he is gaining an edge by breaking the rules. How do I get someone to stop doing this and help him see how he is making the game experience worse for other players?