Cancellation Planet Fitness not possible

I would like to cancel my gym contract with Planet Fitness. At gym they told me to cancel via email which I did. An automated email was returned: "Thank you for your mail, one of our service agents will be in contact with you within 72 hours". Never happened. Ive sent another 2 emails and received same automated return emails. So how is it possible to cancel the membership if this is a one way communication? At gym they dont want to have rights to cancel a membership but signing up is not a problem there.