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). 

To set this up, go to the:

1. Drop down menu
2. Manage my store
3. Revenue tools
4. Click "View" next to Membership
5. 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!

The option to make your Membership stream only is available at the bottom of your Membership settings.

Members will see that your Membership is Stream Only before purchasing:

The stream-only status of your Membership is displayed to Members prior to purchasing.