Yes, you can intercept anything, using middleware, for example.
But, for some reason I the ability to disable the "new user registration" UI was already implemented in Sphinx, but it's not. So I think this is a different feature request.
I created that one here:
Thus, one thing is the ability to disable user self-registration (the above feature request).
Another thing is the ability to create a new user from code and ask for invitation (this feature request).
Although if you require e-mail confirmation, even if you create a new user from code, user will have to confirm his e-mail to create the account, thus I'm not sure if this feature request here is still meaningful?