What is the ruling on money acquired from defrauding a girl by claiming spirituality and sending her ruqyah (Islamic incantations) and necklaces in exchange for sums of money, and what is the optimal way to dispose of it, given the impossibility of returning it to the girl for fear of exposing the matter?

You must stop dealing with the girl, even through social media, and inform her of the truth that the money taken from her is still with you.

You may take the price of what you paid her for silver necklaces and the like, and return the excess, unless she gifts it to you.

Merely what is narrated about her stealing from her mother's money does not prevent dealing with her, because the basic principle regarding a Muslim's wealth is permissibility.

You must advise her of the necessity of caution when dealing with those who claim to perform ruqyah, for fear of dealing with charlatans, and to seek out someone trustworthy in their religion and piety to perform legitimate ruqyah for her.

You must be careful not to engage in lengthy conversations with non-mahram women, because it is not permissible to speak with a young non-mahram woman without need, as this is a cause for fitnah (temptation/strife).