Such connections must be removed separately from both sides – there's no automatic "link".
Each service has a record of your Google account ID and possibly OAuth token. You need to use the service's own website to change the authentication method (e.g. add a regular password or another OAuth provider).
For StackExchange websites, go to your profile → "Edit Profile & Settings" → "My Logins".
Unlinking the Google account at services will not revoke the authorization stored by Google.
Google also records the authorization you've given to each service. You can view those under "My Account" → "Apps with access to your account" (this includes websites) and remove them, although of course if you delete the account all authorizations will go away.
Removing the authorization at Google will not delete your profiles on 3rd-party services. (For example, deauthorizing Stack Overflow and trying to log in again will simply show the authorization prompt again, and will bring you back to the same Stack Overflow profile.)