How do you prevent something that you are technically downloading. Just the fact that you are sharing your videos to them means they are downloading that video from you.
You can't allow them to not download it somewhere else if you are sharing it with them. If they are smart they will just capture the video, lol. It technically goes into your temp folder when you are in the browser , so you can easily just sniff the http packets from within a browser and see the url of the video that is streaming. There is just no way around it, , sorry bro!