What you are referring to is the auto-fill. It will suggest any email address you have actually sent an email to previously, whether it's in the address book or not. To delete any, when in the address book click on "Collected addresses" in the left pane. There you can delete any you want. If you don't want the feature to keep saving addresses, click on the gear icon in the upper right corner of the Inbox page, then on Settings. Under Mail settings, you will see the option to enable or disable the feature for either incoming or outgoing messages.