The built-in junk email filter that comes with Outlook 2007 is not powerful enough.
Does anyone know of a free / open source spam filter for Outlook 2007?
I'm not sure if it works with Outlook 2007 (it says outlook 2000/2002/2003) but you should give Spambayes a try. I use it on my outlook 2003 and it works like a charm!