Instructions
Select your username from the "Users" menu to log into your account. Once you are logged in, highlight your username again.
Press the "Triangle" button to open your user account options menu and select "Delete."
Read the message that says "If you delete the user, saved data for PlayStation 3 format software, messages, trophies and other information will also be deleted. Are you sure you want to continue?" Highlight "yes" and hit "X" to confirm the deletion. Wait for the deletion process to finish.