You can remove the link between your Nintendo Account and an external account by going to "Linked Accounts" after signing in to your Nintendo Account.

If you sign in to your Nintendo Account using the login data of such an external account, you have to create new Nintendo Account sign-in data before disabling the link. This can be done by linking your Nintendo Account to a different external account or a Nintendo Network ID, or by adding a password to your Nintendo Account. To add a password, select "Password" after signing in.