The best way to prevent fraud on your webcam site (or any site for that matter) is to pay very close attention to everything that is going on, and to write some software that will help you to do this.
A few years ago, we wrote our own fraud detection software that we use on our own sites, called Ahmad.
http://2much.net/fraud-detection.php it has made a huge difference and has helped keep our fraud and chargebacks very low.
Basically, here is what you want to watch out for.
First of course, let your own processor do all of their usual credit card scrubbing, and don't waste your time arguing with them. Why? Because they are out to protect you. When you lose money due to chargebacks or fraud, you lose only your profit on a membership site, but fraud on a webcam site can COST you money in the form of payouts to your models.
Next, know your customers. A site visitor who spends a lot of money on his first visit to your webcam site is probably a fraud. Why? Because this goes against basic shopper behaviour. A potential customer will take his time exploring your site, bookmark it, and come back again and again before finally deciding to spend.
Next, know your chat models. Check their sales patterns. Spy on them in the various chat rooms to see if they are doing what they are supposed to do, or doing something else. Some chat models will do their show while others just sit and chat - both are fine - but if she is just sitting there, or not even on camera, or playing a video, all while a supposed "customer" is paying for it, then chances are fraud is taking place.
When we wrote our LiveCamNetwork software, we considered all of the above and more. Customer trends that stand-out as potential fraud are highlighted, and site owners have the chance to payout on fraud, "holdback" payouts using the same model as 3rd party processors, and more.
There's a ton of credit card fraud going on out there and knowing more about your site, visitors, customers and chat models is key to keeping chargebacks and fraud to a minimum.