new bool: bSpawn;

public OnPluginStart(  ) {
HookEvent( "round_start", OnRoundStart );
HookEvent( "player_spawn", OnPlayerSpawn, EventHookMode_Pre );
}

public OnRoundStart( Handle: hEvent, const String: sName[  ], bool: bDontBroadcast ) {
bSpawn = true;
CreateTimer( 12.0, Timer_BlockSpawn, _, TIMER_FLAG_NO_MAPCHANGE );
}

public Action: Timer_BlockSpawn( Handle: hTimer )
bSpawn = false;

public Action: OnPlayerSpawn( Handle: hEvent, const String: sName[  ], bool: bDontBroadcast ) {
if( !bSpawn )
  return Plugin_Handled;
return Plugin_Continue;
}