Note that styles are cumulative, so you could just add to your stylesheet:
Code:
.noindent { text-indent: 0 }
And then modify the desired paragraphs so that
Code:
<p class="calibre8">...</p>
becomes
Code:
<p class="calibre8 noindent">...</p>
(i.e., just add "noindent" to whatever class the paragraph has)