Only developer questions concerning the VSTO technology are handled in the VSTO forum. Your question needs to be asked in an end-user venue. You'll find a link to a "general Office" website in the forum's
Please Read First message; please use that resource for end-user level questions.
FWIW you should be able to turn off the preview, but that will turn it off for the entire application, not just the multi-level lists. Click the "pizza button" at the top left, choose Word Options. In the section Popular, deactivate "Enable Live Preview".
Cindy Meister, VSTO/Word MVP