If you want to delete a secondary email account, then all you need to do is go to menu>settings>accounts, select the account and then under the menu should be an option to remove it. If it is the primary account that the phone is registered to, you need to perform a factory reset. It's either under Privacy or Backup & reset, depending on the Android version. Be advised that a reset will remove all apps and login information. If you have any text messages, email or settings that you want to save, please back them up first.