[ReshiramZekrom]

You can do this way:

PHP код:

IsVehicleEmpty(vehicleid)
{
    for(new i = 0, j = GetPlayerPoolSize(); i <= j; i++)
    {
        if(IsPlayerInVehicle(i, vehicleid)) return 0;
    }
    return 1;
}
COMMAND:destroyveh(playerid,params[])
{
    for(new i = 1, j = GetVehiclePoolSize(); i <= j; i++) // vehicleids start at 1
    {
        if(IsVehicleEmpty(i)) DestroyVehicle(i);
    }
    return 1;
}