There are two steps, it seems, to cut down what is shown on that front page without heavily modifying the theme and core files.
Paths are relative to the home directory of simpleSAMLphp.
Edit the
./config/config.php
and add anauth.adminpassword
and also setadmin.protectindexpage
totrue
Edit
./dictionaries/login.definitions.json
and remove the 'university' instructions to something that is more fitting to your business.