It is possible to detach or remove a custom domain that is associated with your site through your Strattic dashboard.
You can do this by following these steps.
Head to the Strattic Dashboard
Click the "edit in WP" button of the site you wish to edit.
Click "Domain" from the left-hand menu. Scroll to the bottom of the Custom Domain page. The "Detach Domain" button can be found in the right-hand corner. Click it.
You need to type the word "DETACH" to confirm that you wish to remove the domain from your account. Then press the red "Detach Domain" button.
That's it, the removal process will start and may take several minutes. Once removed you will be able to attach a new domain name if you wish.