How to stop this Internet SCAM

[markstanden +]

Hello,

I need some advice please

I run a free classifieds service in Ireland are getting more and more users register from Cameroon, with fake names etc, they validate their email address ok to activate their account, then they start placing Too Good To Be True Advertisements!! hoping they will be contacted by unsuspecting buyers, which they have many times!! All these Scammers want are your contact details, name, email, address, tel no etc, obviously this is an identity scam of sorts!!

Does anybody know the best way to combat this? I know i could stop FREE Ads completely, but the free ads is a big part of my business. Other than checking every registered user's IP address, which i know i can do through admin, and then ban the user, but they only go and set up another account with a different email. So its extremely hard to tell the difference to start with betweeen a real user and a fake one.

Can anybody offer any advice on this, is there a way to ban their IP addresses using 68C? Or is there a better way?
Any help greatly appreciated.

Regards
Mark