[biltong]

IF you have your own admin script/login script, then just make a variable in the script called whatever you like, then set it to 1 when a player logs in. Then, under OnPlayerRequestSpawn, check if that variable is set to 1 for the requesting player, and if not, don't allow him to spawn.

Simple

BUT: If you're a scripting newb or have a script you're not comfortable editing or something, search around a little bit and see what comes up, I know of a few scripts that require you to login before you spawn.