Durhamjen
That is exactly what is being done but you cannot attempt to cut off the oil supplies unless you destroy the production of the oil at it,'s source , the distribution of oil which means destroying the vehicles moving it and the roads they travel on and that can only be done by having the capability to destroy those priorities and have the ability to move into those areas where it is taking place.
You simply cannot have it both ways it is a folly to think that simply talking about what should happen , providing a logical way as to what should. be done to stop IS from obtaining funding from oil will result in it happening . It is the unpalatable truth to stop IS oil production and distribution requires the weapons, the strategy and the ability to hit them in Syria.You cannot achieve anything without going into IS held territory in Syria which is now due to having little to no opposition fighting it holding over half of the land mass of Syria, it is huge.
Think what has happened to Syria. Assad holds only a quarter share of Syria, anti Assad rebels in the north hold a quarter share, IS held territory holds the other half both north and south. Without outside help to defeat IS how long could they hold out against IS achieving it's goal to take over Syria to become a caliphate by slaughtering it's way across Syria.