How do I turn off my default catch all email account?

Your default email account is called a catchall account.

By default your catchall email account is "off."

If your catchall email account is set "on" then any email sent to anything@yourdomain.com will go to your default account (if there is not a separate POP account or alias/forwarder set up respectively).

If you would prefer to eliminate the default account, you'll need to first create a POP account or forwarder (such as, your_favorite_email_address@your_domain.com).

Via cPanel:
To turn off the default email account, click on Default E-mails.
On that page you will see the link at bottom, Set Default Address (Catch All)
Click this link, and you'll see Send all unrouted mail for [domain.com] to _____________.
In the box, enter :fail:

This will ensure anything sent to the now non-existent email addresses will disappear (not be saved).