How to Delete User Accounts on Your MacBook
You can delete existing user accounts that you no longer need from the Users & Groups pane in your MacBook's System Preferences. To wipe an account from the face of the Earth, follow these steps:
Click the account that you want to delete in the Current User list.
Click the Delete User button (which is smartly marked with a minus sign).
Mac OS X displays the confirmation sheet.
Note that the contents of the user’s Home folder can be saved in a disk image in the Deleted Users folder (just in case you need to retrieve something). Alternatively, you can choose to leave the deleted user’s home folder as is, without removing it.
If you’re absolutely sure you won’t be dating that person again, click the Delete the Home Folder option (which doesn’t save anything in the Deleted Users folder). You regain all the hard drive space that was being occupied by the contents of the deleted user’s home folder. As an extra measure of protection, you can also choose to securely erase the contents of the deleted user’s Home folder.
Click OK to verify and delete the account. Click the Cancel button to abort and return to the Accounts list.