Even if you accept that the scriptures approve of shunning, you have a hard time proving that the bible ever mentions or suggested shunning as practiced by Jehovah's Witnesses.
The bible doesn't say that any secret committees is to be formed, that three elders decide on who should be shunned, that what three elders decide determines that a person's family and friends have to shun him. What the bible suggests is, that if there is someone is called a brother, but is known to do these non christian things, you would not talk to them. It's quite simple really, two things have to be true, they claim to be a brother and they are KNOWN to do these Christian things.
It says nothing about leaving Christianity and then living a non Christian life. It says that a person has to be "one called a brother". The elders have been known to come after people years after leaving. What is the scriptural basis of that? It says nothing about a formal committeee. "Judicial committee" is not in the bible. Disfellowshipped is not in the bible. Shunning is not in the bible. It says nothing about a Christian leaving a Christian religion for another Christian religion.
I believe the intent, what they are saying, is if you know of someone calling themself Christian, and you know that they are doing these wrong things, then you would not have anything to do with them. It doesn't mean that they do things that your particular flavor of Christianity believes is wrong, it means a liar, an adulterer, things condemned in the bible. Early Christians had disagreements, it didn't mean they were shunned. Therefore It doesn't mean you get shunned because you don't believe some arbitrary group of men (like the governing body). If one could disagree with an apostle, what makes the elders or even the GB so sacred you must obey them?
It doesn't mean you get shunned for celebrating Christmas, that isn't mentioned in the bible. Or birthdays, or voting, or serving in the military or any of dozens of things you can be shunned for by the JWs.
The Jehovah's Witnesses have twisted the scripture to mean something never envisioned by early Christians. It wasn't to used to coerce people to stay in the church, it was meant to stop hypocrisy. It was to be decided at an individual level, when and if you knew of such sin, they were known to do such things. It wasn't an excuse for elders to question a person about their most intimate secrets, to pry, to condemn, to pronounce judgement. What the elders do is evil, they improperly decide on motive, determine repentance and pronounce judgement, which is only between a person and God. As the bible says, all have sinned and fallen short, so who are the elders to judge?
The worst thing about the Disfellowshiping process is that not only do they cut you off from your family and friends, they tell you that Jehovah doesn't hear your prayers. Can you imagine? Is it any wonder some who were disfellowshipped go on to commit suicide? Who are they to presume to speak for God? They are just men who can be wrong. We know elders sometimes sit on committee's when they themselves are involved in serious sins such as pedophilia and long term adultery. How could someone like that be doing anything approved by God? They supposedly are guided by God's spirit, but how could that be if the elder is involved in such conduct himself? What did Jesus say about the prostitute? Let him who is without sin cast the first stone. Those words of forgiveness and compassion mean something, something the Watchtower ignores.
No, the bible does not justify what the Watchtower does, which is to break up families, hurt people who are going through difficult times and shut down those with valid concerns, all to protect the hierarchy of a cult.