How to Remove All Client Subscription Emails from WHMCS

[PSumpter]

Hello,

I tested my website subscription process using several emails that showed up in WHMCS.

I learned how to delete them when going to 

Orders >> List All Orders to Cancel/Delete

Billing >> Transaction List and delete

Clients >> View/Search Clients and delete

But when I tried to test by subscribing to the low-cost trial, the message on my site said, "Email already taken".

What else can I do to remove all my emails without going directly into the database?

I'm trying to use the same emails to run several tests on my site.