I'm not sure if you have found an answer to this now, but in case anyone else has the same problem, here is how I fixed it:
To remove this annoying feature, create a new e-mail message and from the insert menu, select "Quick parts" and then "Auto text". Right click one of the quick parts and select "Organize and delete", this will open something called the "Building blocks organizer". Remove the quickparts containing your name and... Voila - the annoying autocomplete is gone.
(Perhaps there is an easyer way to access the "Building blocks organizer", but the way I described works)
(Note: also looks like you need to have "compose in HTML" (or possible rich text) enabled)
