I usually see people spitting on the road and I get offended by it. How should I prevent myself from getting offended? What should I do?

Look the other way. People get offended way to easily these days.

Your first question 4 days ago was under Mental Health, but none since.
You were concerned about a 'feeling of unknown fear again and again' for the last 7 years!
2nd question was about people gossiping, 3rd was about hypnotizing a dog, and this one is about an ordinary behavior of people and an overblown offended feeling.

You need HELP, my friend! Are you able to study, to interact with students and teachers? Do you feel that you will be able to hold a job?
If there is fear and misery in your daily life, PLEASE get help.

People spit, esp in the southern part and western part of America. They are not spitting at you, and it is none of your business.

If you don't like it, tough, but no reason to be offended.

I think it's gross too, but try not to be overly sensitive to it. As was said, it can be common for some people... just look the other way and continue on you way.