First, check off "None" for unregistered users and "None" for the users you want to block. Bear in mind that the blocked users could regain access by registering a different account.
Second, check off "None" for registered and unregistered users and allow only trusted users to access your DiscApp. This is more secure, but more work for you.
To have access to the None feature, you would need to upgrade to a Premium DiscApp. Click the $ icon on the admin page for information on upgrading. The easiest way to implement none, in my opinion,... more