What is the ruling on a person afflicted with a mental illness who curses Allah and religion, and accesses pornographic and magical websites? Will he be held accountable for that? And how can he be dealt with?

It is advisable to encourage your brother to memorize the Qur'an and to associate with scholars, to adhere to the morning and evening remembrances (adhkar) and those before sleep, to recite Surat al-Baqarah, especially its last two verses, and to repeatedly recite Ayat al-Kursi. Reciting "Qul Huwa Allahu Ahad" (Surat al-Ikhlas) and the two "Mu'awwidhatayn" (Surat al-Falaq and Surat an-Nas) three times in the morning and evening will suffice him against everything. These acts protect one from devils and sorcerers, because reciting the Qur'an and the adhkar shields one from disbelievers and safeguards against devils. Furthermore, isolating oneself from righteous people strengthens the devil's dominion, and legitimate Islamic ruqyah (exorcism) should be performed. As for holding him accountable for his actions, this depends on his level of comprehension and awareness. A sane person is accountable, while an insane person is not. It must be noted that cursing Allah the Almighty is an act of disbelief according to the consensus of scholars. Entering websites of sorcerers and pornography is forbidden, and this applies to a sane, accountable individual.