public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
    if(GetVehicleModel(vehicleid) == 520) // 520 is hydra
    {
        if(GetPlayerScore(playerid) < 1000)
        {
            RemovePlayerFromVehicle(playerid);
            SendClientMessage(playerid, 0xFFFFFFFF, "You are not allowed to use hydra!");
        }
    }
    return 1;
}

public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
    if(GetVehicleModel(vehicleid) == 520 && !ispassenger) //Check if the vehicle is Hydra and he's not passenger
    {
        if(GetPlayerScore(playerid) < 1000) //YOUR variable here, I dont know your variable, so I use this
        {
            RemovePlayerFromVehicle(playerid);
            //send message or whatever
        }
    }
    return 1;
}

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate & PLAYER_STATE_DRIVER) //If the player entered as a driver
    {
        if(GetVehicleModel(GetPlayerVehicleID(playerid)) == 520) //If the vehicle is a hydra
        {
            if(GetPlayerScore(playerid) < 1000 || gTeam[playerid] != ARMY_PILOT) //If their score is less than 100
            {
                RemovePlayerFromVehicle(playerid); //Remove them from the vehicle
                SendClientMessage(playerid, -1, "You must have 1000 score and be an army pilot to be able to use this vehicle.");
            }
        }
    }
    return 1;
}