Here is the replay from Amazon's support on that topi. I think you will find it helpful:

Hello from Amazon Seller Support,

In order to be able to close your account, your payment balance should be 0.00€.

If you do not change your subscription and remain a professional seller, you will be charged again and will not be able to close your account.

So I invite you to cancel your professional services subscription now on this page:

Settings -> Account Info -> Downgrade Account -> Change Sales Plan or Close Your Account

https://sellercentral-europe.amazon.com/help/hub/support/browse-GB-your_account-change_selling_plan_or_close_your_account?ref=/cu/contact-us?categoryId%3D30003%26typeId%3D30011%26ref_%3Dmacs_xxdwngrd_cont_acinfohm&httpReferer=https%3A%2F%2Fsellercentral-europe.amazon.com%2Fhz%2Fsc%2Faccount-information

Then, once all the mandatory conditions are met you will be able to close it.

For your information, the mandatory conditions are:

1. Have a valid and verified seller account (verified identity and bank information). An account that is blocked or pending verification cannot be closed.

2. Have a balance of payments at 0.00€

=> You must therefore pay a negative balance or have a valid deposit account to receive the positive exedent: If the deposit or payment account is not valid and verified you will not be able to close your account.

3. Do not have inventory of products in Amazon warehouses: if this is the case you need to contact the FBA team to retrieve your inventory

https://sellercentral-europe.amazon.com/help/hub/support/browse-GB-fulfillment_by_amazon_fba-inventory_removaldisposal_orders

4. Do not have orders for 90 days and have no outstanding orders or disputes for 90 days.

If this is not done yet I invite you to deactivate your markets on this page:

https://sellercentral-europe.amazon.com/hz/account-info/vacation-settings?ref_=macs_gvacatn_cont_acinfohm