Offboarded employees stay on your Employees list so their enrollment and billing history stays intact. To take them out of view, open the Status filter at the top of the Employees page and check only the statuses you want to see, like Enrolled, Invited, or Not Invited. Anyone marked Offboarded or Offboarding Scheduled won't appear in your results until you check those statuses.
If you need a record removed completely, email support@thatch.com with the employee's name and we'll handle it. That isn't something you can do from your dashboard.