Have you tried the cover options in the conversion settings?
epub output: No default cover
epub output: No SVG cover
and
docx input: Do not try to autodetect a cover from images in the document
My guess based on no specific knowledge is that you want "No default cover" and "Do not try to autodetect ...". I further suspect that "No default cover" is the answer.
In addition to setting the options in Preferences, be sure to check the options when you are doing the convert as these override the general options set in Preferences and are remembered the next time you convert the book.