The Soufan Group has a pretty good analysis of why cutting off the Islamic State’s financing is going to be difficult.
Summary followed by link to full report:
• Attacking the finances of the so-called Islamic State (IS) with limited collateral damage will be orders of magnitude more difficult than attacking its military factions
• The group has thoroughly embedded itself into local and regional economies in Syria and Iraq, and damaging its finances while not devastating civilian populations will be as difficult as it is necessary
• IS oil revenues might be the easiest to disrupt but such action comes with significant collateral economic damage, while taxes, tolls, extortion, and food sales generate more income while remaining highly resistant to external forces
• In the areas under its control, IS has been providing social services as well as delivering levels of fuel, electricity, and food to populations utterly without recourse, meaning the group needs to be replaced and not simply removed.