[thetwistedpanda]

The easiest way to control this is to write your own "jointeam"/"joinclass" command and completely manage when player's join teams.

*Edit* Actually, all you need to control this is a timer, a boolean flag, and hooking joinclass. Set the flag to true on round_start, the timer is the length of time you want players to be able to spawn (at which point you set the flag to 0), and if the flag is true on joinclass, force the player to spawn.