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)
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Click on "Download Restrictions" and enter in the username(s) of the users you wish to block (image)
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You can also remove users at anytime.
- By default, "Guests" are always disabled.
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You can also remove users at anytime.
Read the official announcement of this feature here.