simpleSAMLphp Authentication plugin and WP login form

If I log into my site via the URL: blog.example.com/wp-login.php simpleSAMLphp opens up, I log in, and I am redirected to my dashboard.

But if I try blog.example.com/wp-admin, simpleSAMLphp opens up, I log in, but this time the WP native login form shows up. It does not matter what username I enter (empty work also), it lets me in since I am already authenticated via simpleSAMLphp.

How can I prevent the native login from showing up?
It is quite confusing to our users.