I'm going to be rude to a guy trying to talk to me on the street
Is there a reason you would be so mean to him?
Because I find him to be too nice.
That is not a polite way to act. It's wrong to be rude to others. You never know what a person is like if you don't stop and get to know them.
That is his problem for following me.
He could be asking you an innocent question like directions, you shouldn't be rude.  How would you feel if you needed help and people treated you like crap in a strange place?