Many thanks for your reply!
Just to update, incase anyone else wants to do this, I created a new phrase called
your_signature_rulesreminder with the message in bold which I wanted to display.
In the parent styles, under 'Modify User Option Templates,' go to 'modifysignature.' Then find
Code:
<!-- message area -->
<div class="smallfont">$vbphrase[your_signature]:</div>
$messagearea
<!-- / message area -->
and replace it with
Code:
<!-- message area -->
<div class="smallfont">$vbphrase[your_signature_rulesreminder]</div><br />
$messagearea
<!-- / message area -->