calibre generated EPUBs from DOCX files do not "have wrong headers" whatever that means. I suggest you simply tell whoever is complaining to educate themselves about EPUB.
And there is nothing wrong with
Code:
<p class="somfink"> </p>
although as itimpi pointed out I seriously doubt those were generated by calibre. DOCX conversion do take class names from docx style names, but I highly doubt there is a docx style named "somfink"