settings: only social connect users may register

I like how Social Connect works for enabling users to register easily using their Facebook and Twitter accounts to enable commenting, while thwarting spammers.

However, this requires requiring enabling the “Anyone can register” membership setting in WordPress. Unfortunately, this setting also allows spammers to register using the ordinary new user mechanism without a social network account.

It would be nice if Social Connect could add a setting of “only social connect users may register”, so that we could enable users to register with a social network account, but disable the ordinary new user mechanism, so as to reduce spam registrations.