SanJ,
Since the text seems to be quite random and email addresses appear legitimate banning on a per-string basis might not be optimal in this situation. Something like this, added to the top of signup.php, can work:
if(strstr($fname,'index.htm')) exit();
You could also ban based on IP as mentioned in
this post.
What I suggest trying instead (or in addition) is modifying your ListMail form to submit with JavaScript instead of the standard HTML submit. Replace the submit button with a standard button with an onclick event as follows:
<form name=sup id=sup method=post action=http://example.com/mail/signup.php>
<input type=hidden name=list value=1>
Name: <input type=text name=fname>
Email: <input type=text name=email>
<input type=button value="Subscribe" onclick="document.getElementById('sup').submit();">
</form>
Please let me know if I can be of further assistance!