There are a few different ways you can delete your Adobe Portfolio.
If you want to stop using all of the services associated with Adobe Creative Cloud, you can cancel your membership at accounts.adobe.com and your site will come down when your membership doesn't renew.
If you want to start over with a Theme, you can go to myportfolio.com/themes while logged in and click "Restore to Default Theme" for the Theme you want to use – this will delete your Portfolio customizations but not your content and Pages.
If you do not want your site to be online, log into MyPortfolio.com and begin to edit your portfolio. In the editor, click "Settings" and then click "Unpublish Portfolio". In that popup you have a few options--you can either display a message or take it down immediately. If you unpublish, keep in mind that your site URL will redirect to myportfolio.com until your DNS settings are changed.