Copy Protect a Digital Version Vs. DVD or Bluray

We produce a lot of DVDs and Blu-rays for our customers.  More and more are starting to ask for digital downloads, however, we need to be able to protect them from passing around the digital copy as our sales come from people purchasing the copy.  So if we sell an unprotected digital file and it is passed around we will loose sales.  Has anyone come up with an option or are aware of an option to sale digital downloads that are protected from being able to be passed around?

Thanks!