[kadet.89]

A way would be to upload a file with steamid to every player (there is an extension for that purpose). When a player connects to the server, download the file back and check the steamid from this file. That can help if the player uses the same game files on all accounts.
There are other ways to distinguish different players by client side key settings (OS, screen resolution...), how player mouses move, player habits. Some players switch weapons more often then others, some press ctrl while shot, some tend to use A and D more often, some have specific binds. There are tons of such small things which can give out any player.