You cannot block the fake clicks from happening, but the big engines such as Google, Bing and Yahoo have multiple security measures in place to prevent you from getting charged for those clicks at all. If you do spot suspicious traffic patterns that may have slipped through their filters, you can send your data over to them, and they will investigate it further and often credit your account. So if someone does try to send a bunch of fake traffic to your paid ads, not only will it not hurt you, but it may even possibly help your site.
Think of it this way, if they are sending bunch of fake traffic to your site and you aren't getting billed for those clicks, other services out there measuring the popularity of your site may think your site is becoming more popular due to the influx in traffic. In addition, technically speaking, a spike in traffic on your paid search ads would also thereby increase your click through rates on those ads. Higher click through rates mean an improved Quality Score with Google/Bing. Better Quality Scores means you have to bid less to maintain a given position. I'm not 100% sure if Google and Bing ignore invalid clicks for the purpose of click through rate, but if they don't ignore them for that, then it means you are benefiting from it whenever someone tries to smack you with click fraud.
