The Ability for Superadmin to sign in as user and being able to read private messages is in many countries illegal even having the technical Possibility to do such can be prohibited by Law. Before using this check the Laws in your country.
If you want to disable it delete Lines 382 to 402 from concrete/single_pages/dashboard/users/search.php
or unzip the attached zip file and copy it to your root directory without changing the structure this will keep the core untouched.