Please add option Verify Mail before registration , and during update profile information (changing mail for members from usercp), no member will be registered , or admitted to the database (both in group waiting activation or any other group), unless the mail is true. As if there was not a failed attempt to register.
What are you trying to achieve with this? Even if this was implemented, entries would still have to be added to the database as instead of a full registration, essentially the email address would be registered and marked as not being verified.
If you can describe your use case, might be able to come up with alternative suggestions to achieve it.