Billing on De-enrolled learner

Despite my notification of the child's de-enrollment in October 2025, Meridian High School in Polokwane re-enrolled my child. They were able to stop the invoicing after five months of confronting them with proof, but they kept charging interest on the account they were supposed to cancel. Please explain why paying interest would be necessary if I had communicated the de-enrollment earlier. To make matters worse they do not answer calls.