Never get caught alone:
Surround yourself with friends and acquaintances when you have an idea that the person is around. Tell your friends to ignore him or her and come to save you when they find you in a difficult spot. Avoid places where you are bound to end up with them alone. Let close friends and family know that you are on an ignoring spree and they should help you with it.
Act as if they do not exist:
Even when you see the person head on, act as if you never saw him or her. Do not acknowledge him or her, even in bitter terms. When you see them approaching, just try to get busy with your cell phone or papers. A wireless ear phone can also save you, as you can pretend you never heard them calling out your name.
Many phones have the block option in them. Put their number on that list. Do the same for internet email addresses and social networks. However, if you want to play smart, do not do any of these and just IGNORE any approach they do through these networks. Do not reject calls. They will know you are pissed. Just ignore. If you have to correspond with that person inevitably for personal or professional purposes, avoid a face to face contact and limit it to emails and messages.
Finish anything mutual:
If you have mutual activities like club or library, alter the timings or try to change locations. Mutual friends are not that easy to remove. Let mutual friends know that you are not comfortable with that person and should be excused from any joint event.