User verification is fine but it does not stop unwanted access in the first instance. This is important as many people would like to use this software in a controlled access context I.E. in lieu of a group forum.

Is there a way to add final admin approval for users after verification? This would require an email to the admin informing them that accounts are pending.

or

Is it possible to create a plugin (similar to the Mobile Number) that adds a code field to the registration form. The code would be set, and easily changeable, in the admin area. Completion of the registration form without the code would return an error message informing the applicant that they should contact the admin.