Regardign the problem quoted, it is more often than not a case of someone changing their email address and either entering it incorrectly and therefore not receiving the confirmation email or else not even answering the confirmation email once they receive it. Also, if they forget their password and and don't respond to the change password email the same thing will happen.
There is a script that looks for users who have not responded to their confirmation email and if so, it sends out 3 reminders over a period of 10 days. If no response after 10 days the Username is automatically deleted. End of story.
If that doesn't answer the problem, please ask users who have been using the same Username for nearly seven years why they don't seem to have a problem. There is no other reason why a Username becomes 'unaccessible'.