Is it permissible for a university student to refuse to photograph their lecture notes for classmates who do not take notes, despite being able to do so?

You are not required to enable your colleagues to transmit the lectures you write, and there is no sin upon you in preventing them, especially if the intention behind it is to encourage them to strive and benefit. However, it is preferable not to prevent them from doing so, and to intend by it the Face of Allah, due to the benefit and assistance it provides to others.