This is technically very difficult to do, if you want people to be able to watch your videos, then the video data has to be transmitted to the end users computer.
Using video streaming software (such as our Lecture Capture Live addon) to deliver your videos instead of downloading the video directly will make it much more difficult for a video to be easily copied, but it will not completely block downloading.
This will require users to disable the RealPlayer plugin before they are allowed to proceed. It will not prevent the downloading of the video by other means. You should ensure that you are not using any RealPlayer video sources in your presentations before enabling this capability.