Pretty much every DRM measure that's been cooked up has been defeated almost immediately. One of the fundamental aspects of security is that there has to be a way through it for the legit users, and if one person can go through it, all of them can. DRM just pisses off paying customers who have to jump through hoops to use the product they payed for, and mildly inconveniences pirates.
Sorry, that's just the way it goes. The good news is, the people that are inclined to pay for something usually still will, even with no DRM.
If you do want to try going the DRM route though, I think it's doable through the Amazon marketplace, they have about as good a system as you can expect (i.e. mostly useless). There's a Calibre plugin that blows right past it.