You can remove an online account which you no longer wish to use.
Many online services provide an authorization token which GNOME stores
instead of your password. If you remove an account, you should also revoke
that certificate in the online service. This will ensure that no other
application or website can connect to that service using the authorization
for GNOME.
How to revoke the authorization depends on the service provider. Check
your settings on the provider’s website for authorized or connected apps
or sites. Look for an app called “GNOME” and remove it.