How can one reconcile reading Ayatul Kursi, which prevents Satan from approaching, with the the Prophet's (peace be upon him) hadith that mentions Satan tying three knots at the back of a sleeping person's head?

The two hadiths are authentic, and there is no contradiction between them. The first is the hadith of Abu Hurairah, may Allah be pleased with him, regarding Satan tying three knots at the back of a person's head when they sleep, and the possibility of untying them by remembering Allah, performing ablution, and praying. Scholars differed on whether these knots are literal or metaphorical, but they are a real occurrence aimed at discouraging the Muslim. The second is the story of Abu Hurairah with Satan, who taught him that reciting Ayatul Kursi before sleeping protects a person from Satan until morning. Scholars reconciled the two hadiths by explaining that the hadith about Satan's knots is general, but it is specific to those who are stated to be protected from Satan, such as those who recite Ayatul Kursi before sleeping.