Finally, a few months ago, I found a way that worked 100% of the time. This raw text file shows what I'm up against, containing all server variables and full text of every comment I've gotten in the last couple of months.

Here's the code for the comment form below. Can you spot my solution? (No, it's not the "http:" part, which almost worked)

    <div class="roundedcornr_top_473174"><div></div></div>
      <div class="roundedcornr_content_473174">
        <div id=commentBox class=commentBox> 
        <div class=pad>
        <h2>Post comment</h2>

        <form action="/blog/index.php" method=POST onsubmit="return validateCommentForm(this);">
        Real Name: <input type=text name=displayname /><br>
        <span style="visibility:hidden"> Your Email (Not displayed): <input type=text name="email"/></span><br>
        Text only. No HTML. If you write "http:" your message will be ignored.
        <br>
        <textarea cols=60 name=comment rows=10 wrap=soft ></textarea><br>
        <input type=submit value="Post" />

        <input type=hidden name=id value="75">
        </form>
        </div>
        <div class=comment>
        </div>
        </div>

      </div>