//the function
COMMAND:cleanv(playerid, params[])
{
new TotalVeh=0;
for(new vehicles=0; vehicles < MAX_VEHICLES; vehicles++)
{
    if(IsAnyPlayerInVehicle(vehicles)==0)
    {
        DestroyVehicle(vehicles);
        TotalVeh++;
    }
printf("%d Vehicles Cleaned!",TotalVeh);//you can change it to return the msg to the player
}
return 1;
}
//The stock
stock IsAnyPlayerInVehicle(vehicleid)
{
for(new playerids=0; playerids < MAX_PLAYERS; playerids++)
{
    if(IsPlayerInVehicle(playerids, vehicleid)==1) return true;
    else return false;
}
return false;
}