Shoppingclubs like fab often offer their members a bonus for inviting friends to the site.

When 10 people register to fab , you get 30$ worth credits, which you can spend on the site. The invited people don't have to buy anything.

How can they prevent people to write a script or register manually with multiple accounts just to get the money?

They don't use captchas. Is there anything else besides IP and Cookies scan?