You can block users from downloading your content from your Account Settings page. Below are screenshots that explaining the process.
- Click on your name and go to "Account Settings" (image)
Click on "Download Restrictions" and enter in the username(s) of the users you wish to block (image)
You can also remove users at anytime.
- By default, "Guests" are always disabled.
- You can also remove users at anytime.
Read the official announcement of this feature here.