Contact Seller Support, and your request will be routed to the VAT Services on Amazon team who will provide the next steps.
In general, you are required to notify the Tax Agent by the 27th of the month prior to the filing window to unsubscribe from all of your countries for VAT Services on Amazon.
Once the VAT Services on Amazon team has confirmed your request, your tax agent will then send a letter to the relevant tax authorities stating that they are resigning as your tax agent.