As I know multi-accounting means that one person open multiple account in series, one after another when previous account was blocked or limited somehow to make bets. But you said that there are a lot of such accounts and they all bets at one time. So, it is can not be one person. And if it is not one person, then this is not multi-accounting. In this case, your claims look strange. People just make bets at your site according the events that you offer. Do you think such actions is suspicious? Return them money and ask do not to play on your site anymore. It is normal practice for many sportsbooks. No players - no problems.