Can’t Disable the Feature 'Administrators Can Login as Any User' in Sandboxes

This is working as designed.

• When a sandbox refresh is completed, a user has login access for 10 years after the refresh date under the following conditions:

• the user is a Salesforce administrator
• the user is copied into the sandbox from the production organization and not created directly in the sandbox

• The above login ability is available to any System Administrator within the Sandbox, regardless of whether the Administrators Can Login As Any User setting is on or off. It cannot be disabled.
• To log in as any user, access your sandbox via test.salesforce.com. NOTE: The option to log in as any user is not available when users access a sandbox from production by using the Login link.