We block you as we want to keep single user accounts on users, making administration and searching for users simple. But what to do when in fact you removed an account?
Simple: On the users page, there is a "Show deleted users" option:
Click that, and you will see you can find your deleted account back!
When clicking the user, you will have 2 options for you: restore and delete permanently.
There you go! You can now RESTORE this user and e-mail can be used again. If the user forgot their password, they can simply request a new password, and they are good to (re-)start again!