They could be from google or other bots if your page has guest access enabled but they will only appear as guests. There isnt much to stop that sorry as bots dont always obey robots.txt for spidering. What Ive done is create a link to the fourms from the homepage using javascript (since the forum is javascript, you have to have it running). I did something like this (as bots cant do javascript links)
<script language="javascript1.2">
document.write '<a href="for' + 'um/default.asp">forum</a>'
</script>