[Scenario]

pawn Код:

CMD:despawnvehicles(playerid, params[])
{
    new
        Iterator:UnoccupiedVehicles<MAX_VEHICLES>;
    
    foreach(new i : Player) Iter_Add(UnoccupiedVehicles, GetPlayerVehicleID(i));
    
    for(new v = 0; v < MAX_VEHICLES; v++)
    {
        if(!Iter_Contains(UnoccupiedVehicles, v))
        {
            DestroyVehicle(v);
        }
    }
    return 1;
}

A little searching is all you had to do. You'll need y_iterate (part of the YSI library) to use this.