How to prevent customers from taking advantage of BOGO deals by returning one of the items for a refund?

You need to employ a multi-faceted approach. Implement strict return policies for BOGO items, clearly communicating the refund structure.

State that if one item is returned, the refund will reflect the discounted price. Consider using store credit instead of cash refunds. Communicate these policies on the checkout page to manage customer expectations and discourage abuse.