What you are talking about is PPC fraud, an increasing problem for businesses on the Internet.
Although at first there seems little one can do to stop this from occurring, there actually are some solutions. First, contact your PPCSE and tell them of your suspicions. Your reports should be able to back up and verify if click fraud is being perpetrated by the repeated number of the same IP address (sometimes reaching thousands of clicks per day from a single IP address).
At times, though, click fraud is a lot more sophisticated and more difficult to catch, but there are many tools in the market, many of which are reviewed on this website, that can also help you identify this problem and work on ways to solve it.