people whom you want to avoid

If that person havent seen you yet, change your path. If you've been noticed, keep talking as minimum as possible. I usually do not stop to say hi, but nod them with my head as a salute. If they say hi/hey just say you're in a hurry and will catch them up later. After a few meets like that, they'll understand (hopefully) that you ignore them. And there wont be much trouble.

Still continue to be polite because being rude is not the name of the game and you wouldn't want to have an enemy or that person to think you have gone crazy or over reacting. It doesn't mean that when you don't like a person you'll treat them badly. Exchange little words as possible and keep track of what he or she is saying and when you really can't bear it all, use excuses like, you really need to go because a friend is waiting for you and you're really late. Be realistic as possible.

well if that's the case then don't avoid that person anymore.instead, better face the reality and talked about that stuff that you're not comfortable with. if we don't know how to face our fears to any situation then we can't readily sleep at night.