The tool that prompts you with an email address that you type in, is called Autocomplete. In 2007 you can delete an individual entry by using your arrow keys to navigate to it in the list and then by hitting the delete key. If you want to turn it off, you can find this feature in Tools >> Options >> Preferences (Tab) >> Email Options >> Advanced Email Options and then untick "Suggest names while completing To, CC and BCC fields". This should also work in earlier versions of Outlook. Let me know how you go.