You don’t get to assume anything; a customer has (politely) written to you that they cannot accept the terms of your license and would like a refund; this is not unreasonable.
What is unreasonable is assuming that this person has criminal intent, and the overly aggressive nature of your response. You are entitled to decline a refund if you don’t believe the person requesting it has a valid enough reason for wanting a refund, however the words ‘lawyer’ and ‘criminal prosecution’ really shouldn’t appear in the response.