http://searchengineland.com/070807-075707.php - it talks about using "geotargeting to improve your ROI, the fact that you can exclude IP addresses from seeing your ads, the fact that Site Exclusion allows you to block an unlimited number of sites, and Google’s invalid clicks report." http://blogoscoped.com/archive/2007-08-06-n58.html - a basic instruction on using Adwords and how to check the return-on-investment (ROI) for individual sites.
I've said it before, and I'll say it again. Don't try to maximise your ROI on Adwords - it's profit you want to maximise! ROI should be used to choose between marketing strategies, not to optimise them. Think about it. In Adwords, to improve your ROI, all you have to do is cut your bids. Since your conversion rate is generally unaffected by your advert's position (if anything, it's higher in lower positions), your cost per conversion will be lower, then less you pay per click. And hence your ROI increases. Using ROI, you can optimise your campaign down to nothing. ROI should be used to determine the most effective thing to do with your money (Adwords, affiliate networks, stick it in the bank). But once you've commited £x ($x in USA) to Adwords, the objective has to be to make as much money as possible from it. Yes, you need a certain minimum ROI to justify putting the money into Adwords, but you certainly shouldn't be trying to maximise it. Apologies for the tone of this rant - I'm buying a new house, and I'm feeling a bit stressed...