new MYCAR;

public OnPlayerStateChange(playerid, newstate, oldstate)
{
  if(newstate == PLAYER_STATE_DRIVER){
    new pVeh = GetPlayerVehicleID(playerid);
    if(pVeh == MYCAR){
      RemovePlayerFromVehicle(playerid);
      SendClientMessage(playerid,0xFF0000AA,"You can't drive this car");
    }
  }
  return 1;
}

public OnGameModeInit()
{
  MYCAR = AddStaticVehicle(...);
  return 1;
}