How can I delete a user's email address so they can sign up for a new site with that same email

I am testing and would like to be able to run through the signup process with the email address, but of course it says “That email address has already been used”. So I went under the list of users in my dashboard, found it, and deleted the account. But I still get the same “That email address has already been used” error when I try to use that email address again.

I know when the site is live and running I won’t want to be doing this because if a user has created an account already, they will just make new sites from that same account. I get that. I’m just asking for testing purposes.