I could make use of a hack that prevented search engines from parsing threads within certain forums. Reason being, some threads contain content that the authors do not want people to be able to find in search engines.

For example, at The Webmaster Forums we have a Website Critique forum. People post their site asking for reviews. We'll, we've had members notice that searches are returning results pointing to our forums where they were asking for help... not very desirable. :(

Please look here and read #B3, it might be what you need: http://www.google.com/webmasters/3.html#B3