I believe that shunning is scriptural but only within the context. I agree with the posts already, that one must be a brother and it doesnt apply to people who have left but also in taking a look at Pauls words in 1 Cor 5. Within the culture of the first century, it appears from history and also scriptural points that the meetings back then obviously were not conducted as today and especially the way the witness perform them. They were used as times to share a meal or as Jude put it a love feast. Even within the pagan culture eating at temples or religious ceremonies was common it was a PART of their worship.
So when Paul said about not eating with a person he MAY have been talking about not recognizing him/her as a brother and especially within the congregational arrangement, but to have normal daily association as a HUMAN BEING was considered Christian as Christ himself set the example eating with the sinners and tax collectors and prostitutes. So to invite a "brother" to a congregational setting and eat with such a person in a congregational setting as they were advocating or trying to bring "leaven" into the congregation would be wrong, but not to be normal, loving to others and especially to those related to us who have left the ranks would be "unchristian" to say the least.
The other scripture that they twist and use in conjunction with 1 Cor 5 is 2 John, again taken completely out of context. In reading the context John was saying not to say to a greeting and invite one into your home (where most congregations formed and met) who was a anti_Christ, a person who denies Christ existence, not saying a greeting to THEM, not having them into your home and perhaps the influence of the congregtion to spread their false teaching concerning the Christ. It has nothing to do with a former witness or brother who leaves and never was it meant to handed down to family members. Where one cannot do the "natural" acts of love toward either children or parents. It goes completely against the scriptures and showing honor to parents and love naturally our children.
So where I believe in shunning, it is to be done with love but only in the context of keeping the congregation clean against a person who is willfully trying to contaminate it, not the way it is used as tool to scare people in submission as the Witnesses do to keep people from leaving their org.
Respectfully
abr