You can change your Membership service to "Stream Only" which will mean purchasing members will not be able to download your vids through your Membership, even if your vids are not individually set to Stream Only (to find out how to set specific vids to Stream Only, click here). Members will be able to see your Membership is Stream Only before purchasing.
To set this up, go to the Dashboard > Quick Links> Membership.
At the bottom of the Membership section, there's a box that says "Make all membership vids Stream Only". Check the box, hit save and now your Membership is Stream Only!