The possibility of supporting pre-registration hooks for other types of connection, like social authentication providers, as been discussed and to my knowledge it was considered for future implementation. However, at this time I can neither confirm if that will indeed be the outcome or any definitive timeline for it.
In relation to the scenario you mentioned, to my knowledge, there would be no automatic or immediate blocking of users and the account would just be flagged as exceeding the subscription quota. However, if you want to be sure of all the details related to any impact on the subscription you may want to consider contacting sales directly.