The Permitted list.txt (also known as Whitelist) allows you to limit the access to your server to only the players added into this file.

Once you have gotten your STEAM_64_ID insert it into the text box, then click to Save Changes.

EDIT: Wait, I just re-read the question. Are you asking to keep only players you know from entering the server? Or if you can do, say; Permanent character type binding to the server? Apologies.