Questions on StackOverflow are being viewed in HTML not plain text. They are sanitized using Jeff Atwood's HTML sanitizer and then converted to HTML using MarkDownSharp.
I asked this question a few weeks back and the solution I ended up with was to store the raw markdown in the database and then transform it when it's shown to the visitor.
Here's how I'm sanitizing my Markdown
''# Because some people can be real ass holes and try to submit bad data (scripts and crap)
''# we have to modify the "About" content in order to sanitize it. At the same time, we
''# transform the Markdown into valid HTML
user.About = Trim(Utilities.HtmlSanitizer.Sanitize(MarkDownSharp.Transform(user.About)))
Since MarkdownSharp is open source, I'm sure you could dig into the source code and remove the additional tags that you don't want to see in the preview.
EDIT:
Since in my example I'm sanitizing the HTML before converting the markdown, I think you would have to remove the <b>
or <strong>
tags in both the HtmlSanitizer and MarkdownSharp. The reason for this is that you'll need to sanitize raw html tags AND markdown tags.