Restrict access to a new schema to a specific role despite users having db_datareader?

I’m working in a SQL Server 2016 database that is not of my design nor can I fundamentally change the security structure. I know it is likely trash/worst practice but I have to play the hand I am dealt at the moment. I’m trying to restrict access to a schema to only members of a role, but everyone seems to have db_datareader membership. Here are the details: