Is it forbidden to reserve a seat in a university course and then cancel the registration in the last few days, which might delay another student's registration?

If the system permits you what you mentioned, then there is no blame upon you for doing it. What you fear of potential harm to others does not necessitate its prohibition upon you. What is forbidden is the intention to harm others, or to inflict harm upon them without just cause. Ibn Taymiyyah said: "Harm is forbidden by the Qur'an and Sunnah. Texts have indicated the prohibition of harming others unjustly. However, inflicting deserved harm out of necessity, not with the intention of causing harm, is not considered harm."