If you usually pay with PayPal, the first contact should be with PayPal support to find out why your payments are no longer processing. Depending on the answer from them, you could move on to contact Jagex Billing Support but Jagex Billing Support usually can't help if they haven't received any payment through from PayPal.
The support page with contact links to both PayPal and Jagex is: Common PayPal Issues note: the Jagex contact is the
I suspect you're paying for membership on a different account.
You can cancel future payments through your PayPal account (look at your subscription settings). If you want to request a refund for any payments that have already been processed, Malua gave you the contact information for Jagex's billing support team.