Refunds, Fraud Problems

Hello,

We detected some fraudulent activity in one of my app. The user purchases the in-app product, makes use of the service, and then returns it immediately. And he repeats this process every day. We have done some actions to prevent this, but this process continues by using clone app creators. How is the return process going, is there no control in this regard? Normally, if a user buys the same product for the second time, I think that he should not return it anymore.