Hi Hans,
Click Fraud is always a hot topic in the PPC universe. There are some things you can do to monitor your traffic for fraudulent clicks, and the engines are always working to improve their technology to detect it.
If you have an analytics program installed on your site, you can check the referring sites that are sending traffic to your store. If any of them are sending large amounts of traffic with no conversions, this is a warning sign. The same can apply to IP addresses sending large amounts of traffic. You can cross check these with the placement reports in Google Adwords that show where your content network traffic is coming from, and you can spot anomalies that way as well. Both Google & Yahoo offer the option to block domains, so it's easy to block sites that are sending click fraud traffic (or just low performing traffic in general). There are also more sophisticated 3rd party tools that can help identify click fraud if you think you have a serious problem.
The major search engines do look for click fraud and regularly give refunds for it (check for adjustments in your billing statements). If you find a possible instance of click fraud, contact the appropriate engine and they will be happy to assist you. At the end of the day, the search engines know they will lose advertisers & revenue if they don't work to combat click fraud & give out the occasional refunds.
