Either put a rule in place that a registrations site can't be deleted if it has at least one applicant or registrant, OR put a rule in place that a registration site can't be deleted if it is active (e.g., they would need to uncheck the active box first.
Either put a rule in place that a registrations site can't be deleted if it has at least one applicant or registrant, OR put a rule in place that a registration site can't be deleted if it is active (e.g., they would need to uncheck the active box first.