You are free to can cancel your seller account subscription at any time. You are not tied into any contract.
To cancel your subscription login to your MadeMe account at: https://mademe.co.uk/login/ .
Then go to: My Account > Membership Details. Then under your current listed membership plan, click the "Cancel" link as in the screenshot below.
When you cancel you seller account all of your products and order details etc. will be deleted and no further payments will be taken.