​How to politely tell people “you already agreed to this policy”?

I run events that require ticket sales, and in the booking process, the cancellation policy is explained and people check a box to say they have read and understood the policy. The policy is also available on our website. We do not offer refunds for last-minute cancellations, and people who want to cancel often seem completely blindsided, and some get irritated. Is there a polite way to say “you already agreed to this policy when you bought the ticket?” Any other suggestions to help with this?