Something

public OnPlayerStateChange(playerid, newstate, oldstate)
{
if(GetPlayerState(playerid) == PLAYER_STATE_DRIVER) {
 if(PlayerHasDrivingLicense!=true) { // just replace this with your own check,
  RemovePlayerFromVehicle(playerid);
  SendClientMessage(playerid, COLOR_RED, "[POLICE] You are not allowed to drive a car!");
 }
}
return 1;
}