I don't think you can stop Calibre adding all the classes to the <p>, <h1> etc tags inside the converted epub, but if you already have a standard css file which you use with your simple html, then those Calibre-created classes should give you the same visual result as your standard css file.
There are several ways to make Calibre use your standard css file:
- Add a <link> to your standard css file in the html header section before you import the html file into Calibre
- Add your standard css statements, between <style>...</style> tags inside the html header section before you import the html file into Calibre
- Copy your standard css statements to the Look&Feel ExtraCSS box as you convert zip-to-epub.
- Type the filename of your standard css file (onyour PC) to the Look&Feel ExtraCSS box as you convert zip-to-epub.