How to Prevent Blocked Callers from Reaching the Auto Attendant in Admin Console

Objective

Prevent unwanted callers from reaching the Auto Attendant in Admin Console

Remove the number in question from the Auto Attendant.
Create a user and add a license to it, in order to be able to assign the number previously assigned to the Auto Attendant.
Go to the Call Forwarding Rules section of the newly created user:
Configure the When the user gets a call from a blocked caller rule.

Set up a new custom rule which always forwards to the Auto Attendant in question.

Make sure that the Block Callers rule comes before (above) the custom Always Forward rule in the Call Forwarding Rules hierarchy.

As the Auto Attendant would not have a built-in blocked caller function, it would need to go through an extension first which can filter out the unwanted calls.
Please note that Call Forwarding Rules prioritizes active rules from top to bottom. This is why a new custom Always Forward Rule needs to be created instead of using the existing Forward All Calls rule, as that cannot be moved, only disabled.
Currently the functionality to block numbers for an entire phone system is not supported in Admin Console.