Like all PPC engines, the traffic available from AdWords suffers from click fraud, low quality partner networks and aggressive competition. These factors can often mean you pay for clicks you don't want or need. Of course Google do an excellent job in cleaning this up, but often poor quality traffic gets through the net. Is there anything online marketers can do to further filter, monitor and clean up these clicks?
Online PPC campaign management can be a full time job, and it's purpose is to optimize campaigns. Basically you spend your time:
a) Deleting unprofitable ads, keywords and audiences,
b) Testing new ads, keywords and audiences,
c) Adopting and investing more in campaigns and ads that work.
d) Improving your website and conversion funnel.
To do your job, you can use Google Analytics (properly configured), Google Adwords and Google Website optimizer (if you want to do A/B tests).
You can also choose to place your ads on Google Search instead of other websites. Usually there's less scams in ads on search results pages.
Finally you have to accept that you can't have 100% of qualified traffic. Hopefully traffic that converts will produce enough to pay the bills.