Easy diplomatic solution:
Detain the GRACE 1 crew in Gibraltar and let them go through due legal process for breaching sanctions, then...
1. Fly out a new crew to take the ship back to Iran, full, or
2. Sell the fuel to a neutral country at baseline market price (without transportation costs), then the second crew take empty ship back to Iran. (But, the quantity of cash being carried on GRACE 1 would attract the attention of pirates around the Horn of Africa area = not our problem!!! - Send your speedboats out there!)
The first crew would likely try to claim asylum, get rejected and be home with family soon enough.