You can stop the link by pressing [登録解除 (cancel the registration)] under [d アカウント連携登録有無 (d account link registration)] on the [基本情報 (basic information)] screen after you log in to "Web Billing."

If you want to link them again, you can link them by pressing [d アカウントでログイン (login by d account)] on the upper right screen of "Web Billing."