I had problems with people scraping Twicsy so hard that it was taking the site down. For a while I would manually review the top IP addresses requesting pages a couple times per day and look for patterns and ban IP's based on that. Then I created a script based on the patterns I recognized to do it automatically.

But then I just made Twicsy fast enough to deal with the traffic so I don't need to worry about it anymore. I guess it depends on your business model whether or not that will work for you.