Sorry I can't find the original thread this question was asked.
Some said something like I wish there was a website where you could look someone up and say no I won't go near him because he dated x y z person and did this this and this and was an arsehole because
I knew there was a site but couldn't remember the name, which I have now
www.dontdatehimgirl.com
you have to have a user name and password to access the database, and its a bit cheesy and american, but the basic principle is that if you meet a bloke who is a liar and a cheat then you can expose him and warn others