An interpersonal conflict situation

This situation happened in my current course of study in NUS. Dipsy(not his real name) was one of my good friends in NUS. However, he had this bad habit of asking questions regarding schoolwork during non-school hours. Many of us would be acceptable to the idea of their good buddies asking for help during school hours. But it gets a little on my nerves as the frequency of his questioning kept increasing the more I help him.

Ok now you guys may think I am being selfish here. Consider the following scenario; you are sitting comfortably at home deeply immersed in your schoolwork. Suddenly, you got a call from Dipsy. After picking up the call, Dipsy asked you to flip to a page of your notes to clarify a concept from the notes. As you were busy with your own work at that time, you offered to get back to his question another time. Instead of agreeing to your suggestion, he pressed on with his question. Even though he did not demand an answer, he was indirectly trying to get you to answer his question immediately. The above scenario was what usually happened when he called me for help in his schoolwork.

The first few times was acceptable and I helped him. Subsequently, his request gets more and more frequent. In addition, it was getting a little irritating because his frequent calls often disrupt my train of thoughts while I was doing my work. He was very persistent to get me to answer his questions immediately. On the account of our friendship, I did not reject him. At some point in time, I felt I had enough and decide to talk to him nicely about it. So the next day in school, I asked him politely if he could to ask his queries during school hours. Asking me during that period was more convenient for me and it was also easier to explain questions to him face to face. He agreed to my suggestion. 

There I was at home feeling happy, thinking that he finally understands my situation. WRONG! The subsequent weekend, he started his query spree again. I started to feel more irritated because my advice had fallen on deaf ears. I tried to think of other methods to curb this habit of his but to no avail. Hence I decide to just ignore his calls. He began to message me asking why I am not picking his calls and I ignored all of them too. A few days later, he did not call to ask me questions anymore. Instead, he clarified his doubts regarding school work with me in school.

It seems like a problem solved! But it had affected our relationship. Sometimes I can feel a little bit of awkwardness when we talk to each other. Anyway, we are still good buddies. I felt that I have not handled this situation correctly, resulting in a little scar in our friendship. Maybe you guys could give me some suggestions on how to handle it better!! Thanks!