Juana's "sixth sense" has already alerted her to the trouble ownig the pearl could bring on, but it isn't until Kino is attacked in the night (and has to kill in self-defense) that her initial misgivings are confirmed. This incident could be considered the point of no return because from this moment on, Kino and Juana are on the run to escape reprisal (and are not just going to the city to try to sell the pearl themselves).
To Juana getting rid of the pearl would mean getting rid of the curse it has brought upon her family, and as we see later, her instinct about this is right. Kino, however, will not take her advice and insists on "winning" against the age-old system and hierarchy of "the rich" and "the poor."
Moreover, after the destruction of their house and their boat, Kino and Juana are not given a real alternative other than to leave. Kino is aware that the curse upon them might inflict harm to his extended family as well. He tells his brother, "I am like a leprosy." Like the venom from the scorpion's sting, greed for the pearl has contaminated the whole community and has alienated Kino and Juana from everyone. They have lost their place in the community, but their greatest loss is yet to come.