Step 1. Log in to the WHM
Step 2. Type list account in search bar and then click on List Accounts.
Step 3. Now type your domain in text field and click on find. Now click on "+" button which is next left to the your domain name. It will display all options related to this hosting account and now look for Terminate Account option.
Step 5. Click on option Yes, remove this account but make sure to have a complete backup before proceeding. Your complete data will loss in this process.