If it is only in your email address box that is auto completing you can turn this off by going into the settings found by clicking on your user name at the top right hand side. Then go to "Contacts" at the left hand side and untick the box to auto complete the email addresses.
If it is in other places such as when you log onto a site it is your web browser that is saving the email addresses. You need to go into your browsers settings and you will find something such as "Passwords" or "Auto Fill".
Some browsers allow you to change/delete the autofill when you go to log on you can right click and a box will open allowing the inserted address to be edited or deleted.
Look down the list and delete the details that you do not want.
