In Word: make sure paragraphs that have a Style of
Heading 1 or
Heading 2 avoid the use any of the words that trigger calibre's
Conversion->Common options->Structure detection->Detect chapters at (XPath expression), which are:
Code:
chapter, book, section, part, prolog, prologue and epilogue
Or don't use Styles
Heading 1 or
Heading 2
In Calibre: set
Conversion->Common options->Structure detection->Chapter mark: to
none - either per book or in Preferences.
If it were me, I'd change the settings in Calibre, probably as needed for a particular book rather than for all books via the settings in Preferences.
BR