Hi there, Thomas! This is Marc. Thanks for reaching out to the community.
The old MSN Dial-Up subscription is separate from your Microsoft account, so cancelling it will not stop you from using your @msn.com email to sign in at Microsoft.com or Outlook.com. Your email address and account will remain active as long as you continue to sign in regularly.
To stop the charges, contact MSN Member Services and request cancellation of the Dial-Up plan. You can find the steps and contact details here: https://membercenter.msn.com/ContactSupport.aspx. You can also check your Microsoft account’s Services & subscriptions page to see if the billing appears there and cancel directly: https://account.microsoft.com/services.
Let me know how it goes.
Best regards,