Collections won't leave us alone - Consumer Complaint for PLANET FITNESS - HEAD OFFICE

I joined the gym only to have to cancel due to an injury. I contacted the gym with a doctor's letter, paid the final bill. All seemed to be accepted by the Planet Fitness. Then I started receiving collection emails. I sent them the relevant information. They do not respond to this but every month they send the collection demand again. This is over a year now and they won't stop. What am I supposed to do????