I would say be direct but understanding, and sugar coat it a little, dont just say "I dont want you back" be gentle.
There is no way to say it so the other party isn't hurt, but remind them that there is a reason this is an EX relationship. Things are no better now, than when you broke the engagement and wish her/him luck in the future. Then walk away. Don't prolong it. What's the point?
A bullet in the head. She won't feel a thing.
no good way other than to tell then the truth
Hearts have to break. There is no way to say this without causing pain.
The LEAST painful way to do it is to do it in no uncertain terms and make it abundantly clear that your decision is final.