Identifying spam text

Hi there, I am running a community site in php, where members can be contacted through a web form. Yesterday a guy contacted about 50 of those members with a spam text. Now I am trying to find a way to identify spam text via php. This looks like a common task to me, so I hope that I have not to invent the wheel twice. Maybe someone knows a good script to do this? Basicly it looks like that the text has to be checked against certain key words and if they reach a certain amount of hits it is spam text. Has anybody a good idea on how to start on this? Thank you for any hint, Merlin