As we have seen, it depends on the preposition which pronominal adverb (meaning either y or en) is to be used. However, this rule does apply only as long as the referred object is not a living person (or animal).
Est-ce que tu penses toujours à Paris? Oui, j' y pense encore.
Do you still think of Paris? Yes, I still think of it.
Est-ce que tu penses encore à lui? Oui, je pense encore à lui. NOT! J' y pense encore.
Do you still think about him? Yes, I still think about him.
NOT! I think about it.