Why does Pip refuse to take any more of Magwitch's money?

When Magwitch returns to England and reveals himself as Pip's benefactor, Pip is completely at a loss for how to respond. His decision to not take anymore of Magwitch's money results from a combination of shame, fear, and possibly a bit of prudishness.

Consider that, at this point in the novel, Pip is confronting the fact that everything he thought was true about his life, his expectations, and his newly created identity is untrue. Miss Havisham did not choose him to receive a great fortune; he is not destined to marry Estella, the love of his life; and his finances are not nearly as stable as he had previously thought. Money from Magwitch is drastically different from money from Miss Havisham. Pip views the money as connected to a criminal past, and he tells Herbert in chapter 41 that he is afraid of the debt he now owes to a dangerous man. Pip is perhaps afraid that he is now under the control of a convict. Additionally, the money is not associated with the glamorous life of Estella, Satis House, and the British upper class but rather associated with the most traumatic event of Pip's childhood and the very world Pip has decided to abandon.

This is largely because, by this point in the novel, Pip has become a snob. Now wealthy and well-educated, Pip is ashamed to be connected to anyone who isn't. We have already seen him, for instance, attempting to dissociate himself from Joe, which he later admits was a cruel and selfish thing to do. Given that Joe raised him (and is simply working-class, rather than a convict), it's hardly surprising that Pip reacts negatively to Magwitch. Although Magwitch claims to have earned his fortune honestly, Pip possibly suspects otherwise; in the first shock of his discovery, for instance, he says that "for anything [he] knew, [Magwitch's] hand might be stained with blood" (Chapter 39). But even supposing that Pip did accept Magwitch's account of events, he would probably still regard his money as tainted simply because of Magwitch's social status and past crimes. When Herbert questions Pip about keeping the fortune, he simply says "Think of him! Look at him!" (Chapter 41).
In fairness to Pip, of course, there is something unsettling about Magwitch claiming him as "[his] gentleman" (Chapter 40). The phrase implies ownership, and Pip probably feels that taking Magwitch's fortune would mean giving Magwitch some kind of power over him; Pip talks, for instance, about feeling burdened by the "gold and silver chains" Magwitch has placed on him (Chapter 39). Still, Pip ultimately comes to regard Magwitch with affection, and to see Magwitch's generosity (though misguided) as sincere, so it seems likely that prejudice was at the heart of his earlier objections. By the time Pip comes around, though, the entire question of accepting the fortune is moot, because Magwitch's status as a convict makes the money forfeit to the Crown.