new ACar[3]; // Change 3 with the amount of admin cars you have
public OnGameModeInit()
{
  ACar[0] = AddStaticVehicle(...); //Admin Car1
  ACar[1] = AddStaticVehicle(...); //Admin Car2
  ACar[2] = AddStaticVehicle(...); //Admin Car3
  //So on...
  return 1;
}
public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
  for(new V; V</*Amount of admin cars*/; V++)
  {
    if(vehicleid == ACar[V])
    { 
      if(ispassenger == 0)
      {
        if(/*Admin Variable here, I'll use IsPlayerAdmin as a example*/ !IsPlayerAdmin(playerid))
        {
          new Pos[3];
          GetPlayerPos(playerid, Pos[0], Pos[1], Pos[2]);
          SetPlayerPos(playerid, Pos[0], Pos[1], Pos[2]);
          GameTextForPlayer(playerid, "~r~This is a admin only vehicle!", 1000, 3);
        }
      }
    }
  }
  return 1;
}