lol there's already a command to disable it.

No he wants to have postspawn enabled, but he doesn't want to have players rejoin the game and still be able to spawn again.

yeah, mp_postspawn controls the time in seconds after roundstart in which new players are allowed to spawn.

just lower the value to make this stupid rejoin action less annoying (something from 0 to 15 secs should be cool, depending on map size). you could also write a little Lua script to protect against stupid re-join trolls.