Good question, and one that I didn't answer in the book (and probably should have). I would say it depends on how cruel your storyguide is feeling. Perhaps the jinn would just grumpily tell you that such a task is beyond its abilities. On the other hand, a crueller storyguide could rule that asking a jinn to accomplish a task that it cannot do results in the jinn shaking off any further compulsion and being free to attack its master.