well, I don't ever want to make the ready nagging optional in the warmup-stage for any of the players (but instead force them to see the nag) because I really know how lazy people can get. People think "yeah, ... well .. I will just disable nagging in general as I am experienced enough and I won't certainly miss the start of the game ..." and so on. But this is just not the case, it would result in too many players having it disabled, and the games would take longer to restart
So I am just talking about the options we have for readying up during match stage. Actually the easiest way would be to simply do what I already suggested: disable the restart-possibility (and by this also the nagging) after the map was restarted for the first time. If the players really needed another restart they could simply vote for a chmap for the same map they were already playing and readying up again. Although I have to confess that this isn't too elegant either.
It is simply a tough decision. Basically one has a feature that causes negative effects that become new problems. Solving these effects can cause new problems (like when I would allow the individual players to disable that they are nagged the game would take longer to start, or when I would decide simply not to show nagging to anyone if someone "accidentally" hit F4 in match stage would also result in a long waiting time until every one readied up again). So the easiest thing would be to disable the feature (which is that one can ready-restart even after the match has begun) which also moves all resulting effects/problems out of the way.