Removing the configured domain (i.e. "@example.com") from the SSO configuration will not remove historical users.
While removing the domain would still need to be done as a step to disable SSO completely, only removing the domain will result in breaking the routing. Logins associated to the "@example.com" domain will result in user emails (ie. "[email protected]") failing authentication with an error message stating "Sorry we cannot log you in at this time"