Dont know if your exact option is listed but you can find the switch for parcferme in rf/userdata/player .plr file
look for this line
GPRIX ParcFerme="3" // 0=off, 1=no setup changes allowed between qual and race except for 'Free Settings'), 2=same unless rain, 3=use RFM default 'ParcFerme=<0-2>'
MULTI ParcFerme="3"
ParcFerme=2 is only good for this situation, i think
one more question, how to close pit after race start? that nobody can leave until leader finish 1st sector? (now waitings can leave pitlane 5~10s after start)
I dont think you can set that time after green to open pits , at least i cant find it in .plr ..?
however you can adjust some of the pit timeings with these lines in same .plr
Recon Pit Open="300" // Time that pits remain open for recon laps (real-life is 900 seconds)
Recon Pit Closed="150" // Time that pits are closed before formation lap (real-life is 900 seconds)
Recon Timer="1" // Whether timer is displayed in message box