www.cssnation.net approached me today asking how they could restrict access to their server based on a file of SteamIDs. There are many ways to do it, but the best way is to use the admin system.
The admin system allows you to create your own lookup tables and custom admin systems. This script uses that feature to read admins from a file and add them into a custom admin pool. If a steamid is not in this pool, the client is rejected from the server.
The file it reads from is sourcemod/configs/allowed_steamids.txt