Use the HTMLparser module, or install BeautifulSoup, and use those to parse the HTML and disable or remove the tags. This will leave whatever link text was there, but it will not be highlighted and it will not be clickable, since you are displaying it with a web browser component.
You could make it clearer what was done by replacing the <A></A>
with a <SPAN></SPAN>
and changing the text decoration to show where the link used to be. Maybe a different shade of blue than normal and a dashed underscore to indicate brokenness. That way you are a little closer to displaying it as intended without actually misleading people into clicking on something that is not clickable. You could even add a hover in Javascript or pure CSS that pops up a tooltip explaining that links have been disabled for security reasons.
Similar things could be done with <IMG></IMG>
tags including replacing them with a blank rectangle to ensure that the page layout is close to the original.
I've done stuff like this with Beautiful Soup, but HTMLparser is included with Python. In older Python distribs, there was an htmllib which is now deprecated. Since the HTML in an email message might not be fully correct, use Beautiful Soup 3.0.7a which is better at making sense of broken HTML.