My teenager has started bringing in ideas from online content that go against Islam. Should I ban it outright?

Do not rush to ban it, because ideas forbidden without explanation often become more attractive in a teenager's eyes. Curiosity is normal, and harsh bans often push teenagers to look in secret. Discuss it openly: first listen to what appeals to him, ask about his reasoning, and share the Islamic view calmly and with sound basis. The Prophet ﷺ said: “Indeed, gentleness is not found in anything except that it adorns it” (Sahih Muslim, Kitab al-Birr wa as-Silah wa al-Adab). If the question is beyond your knowledge, find a trustworthy teacher or reference together. A home that allows hard questions becomes the place a child returns to, not the place where he hides his confusion. See the family & the digital world.