We had a discussion about this a while back, and it wasn't conclusive. See (it's two pages):
backers-cancelling-pledges-t822.htmlThe short answer is, we couldn't come up with an easy/seamless way to do it, unless you've already communicated with them.
"So, I could ask them directly, but only because I took the time to thank each new backer personally as they joined our project. Otherwise, I'd never be able to find them or communicate with them. Once they drop your project, you have to go to pretty extreme lengths to get a message to them. The only way to contact them is to pull up the original message I sent them and reply to it. There is not a way that I have found from the dashboard for project creators to reach these people. I felt that the level of difficulty might be an indicator that Kickstarter didn't want me contacting them."