How do I break up with this guy? Nicely? so that he would stop pestering me?

Try to let him know that a relationship wouldn't work out. If he told you he likes something, claim that you hate it so much and vice versa, turn it into an argue and then apologize instead of telling him in the face and do it several times in different situations. If he's gentle act wild, and If he's wild act gentle! You have to proceed doing that for a month or some time so he won't be shocked. After that tell him that you're different and can't be together. You have different ambitions, thoughts, and beliefs so you will never fit together. He will definitely understand