Here are the steps to remove entries from the AutoComplete cache

• Open a new email message.
• Type the first few characters of the AutoComplete entry that you want to remove.
• When the entry appears in the list of suggested names, move your mouse pointer over the name until it becomes highlighted, but do not click the name.
• When the “X” icon appears next to the highlighted name, click X to remove the name from the list, or press the Delete key on the keyboard.

To disable AutoComplete or clear your AutoComplete completely in Outlook.

In Outlook 2010 - 2016, follow the steps below to have this completely disabled. This is a sort of required in order not to send an email to a wrong recipient.

- click the File menu and 
- select Options.
- In the Outlook Options window click the Mail tab.
- Scroll down to Send messages. 
- Uncheck the Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines box.

Note: To clear the AutoComplete list, simply click the Empty AutoComplete List button. You’ll see a confirmation window about clearing the AutoComplete list, click Yes.

Outlook AutoComplete should be completely disabled and any history that was in the auto-complete list should be completely erased (if you clicked the clear autocomplete button).