You can easily delete any senders you add essentially making those addresses unverified again.
To delete a sender click on Senders in the navigation menu on the left, click on the gearbox icon next to the target sender, and then click Delete.
You can delete both verified senders and those still pending verification.
Deleting a sender will gracefully restart any running (Mautic) instances, so that the address is removed within them. Attempting to send mail via an unverified identity will either be rejected, or potentially flagged as SPAM.