Step 2: In the System Preferences dialog box that appears, click the Accounts tab under the System pane.
Step 3: In the Accounts screen, click on the lock icon. A dialog box appears, asking you to enter the Admin password, enter the Admin name and password, and then click the OK button.
Step 4: In the Accounts screen that appears, you will see a list of accounts on the system, click on the account name to delete, then click the delete button (the button with the minus symbol) under the Login options pane.
It will display the following window, just click the Delete the home folder option and click the OK button. And all data from the related system will be deleted completely.
Source: Trickyways