The auto complete entries are stored in a document with an NK2 extension. You can use a little program called NK2-View to edit the entries in your auto complete. The program automatically locates and opens the document into an accessible editor.
Be sure to open the editor when Outlook is closed. You'll also be asked to back up the file before editing the document.http://www.nirsoft.net/utils/outlook_nk2_autocomplete.html