The Watchtower excommunicates or 'disfellowships' JW's who don't obey the Society's rules.  The rules are written in such a way that anyone can get disfellowshipped if they challenge the authority of God's organization, the Watchtower. 
Jehovah's Witnesses 'disassociate' themselves by having a blood transfusion, supporting another church, getting involved in politics, or resigning from the organization. 
JW's cannot associate with or even say a greeting to disfellowshipped and disassociated JW's. All JW's who leave are treated as wicked, evil people who chose to leave God and are now puppets of Satan. 
Disfellowshipping is judging on behalf of God in order to keep 'God's organization' clean.
Jesus said, 'love your enemies, do good to them, and lend to them without expecting to get anything back. Then your reward will be great, and you will be sons of the Most High, because he is kind to the ungrateful and wicked. ... Do not judge, and you will not be judged. Do not condemn, and you will not be condemned.' Luke 6:35-37
Notice that the Bible is very clear. In Luke 6:25-37 Jesus told his followers NOT to judge the wicked. He recommended that we love our enemies, the ungrateful, and the wicked because God treats everyone with kindness. Jesus didn't mention any conditions whatsoever upon which it is ok to judge, shun, or hate people. He made it clear that God loves everyone without conditions, whether they are good or bad, and his followers must do the same to please God.
The Apostle Paul said, 'What business is it of mine to judge those outside the church? Are you not to judge those inside? God will judge those outside. "Expel the wicked man from among you.' 1 Cor 5:12,13
Notice how the Apostle Paul ran ahead of what Jesus said in Luke 6:35-37 Jesus told his followers not to judge because God's loves the ungrateful and the wicked. Even though Jesus had harsh words for the Pharisees, there is no evidence in the Bible that Jesus refused to talk with them, or hated them. It is possible to believe someone is wicked and still love them the way they are. That's what Jesus was trying to teach. God loves the ungrateful and the wicked. That's why we should do the same. Jesus is a perfect example of God's love for everyone, - not just Jehovah's Witnesses.
The Apostle Paul however suggested strongly that fellow Christians should be judged and tossed out of the church if they are wicked. In the process of judgment and shunning, a growing number of enemies were no doubt created. Why did the Apostle Paul try to improve upon the teachings of Jesus? Did he believe in disfellowshipping?
Could it be because the Apostle Paul was born and raised as a Pharisee?  Could it be that before converting to Christianity he actively pursued Christians and turned them over to authorities so they would be put to death?
Even though the Apostle Paul was no doubt a sincere Christian, he was also a Pharisee and claimed to be a Pharisee AFTER he became a Christian.  Jesus warned his followers about the leaven or yeast of the Pharisees which is hypocrisy, self-righteousness, concern about the law and outward appearances instead of matters of the heart and love. The Pharisees judged Jesus harshly because he exposed the unloving nature of their doctrines.  Luke 11:42 That's how apostasy and a shift away from true Christianity began almost immediately after Jesus died.
Religions today have a choice. They can follow the pure and perfect teachings of Jesus Christ who explained God's love for our 'neighbors' and brothers who might happen to believe differently that we do. OR they can oppose the teachings of Jesus and go with the teachings of the Apostle Paul who was a self proclaimed Pharisee and persecutor of Christians prior to becoming a Christian. 
Who will you follow? Jesus? Or the Apostle Paul? True Christians follow Jesus because only Jesus has the 'words of eternal life' John 6:67-69 
Here's what Jesus said about the Pharisees. 'Meanwhile, when a crowd of many thousands had gathered, so that they were trampling on one another, Jesus began to speak first to his disciples, saying: "Be on your guard against the yeast of the Pharisees, which is hypocrisy.' Luke 12:1
'And Jesus said to them, "Watch out and beware of the leaven of the Pharisees and Sadducees." ' Matthew 16:6
"How is it that you do not understand that I did not speak to you concerning bread? But beware of the leaven of the Pharisees and Sadducees." Matthew 12:11
Should Christians follow Jesus, or the Apostle Paul and his early Christian congregation? Religions can do whatever they want but they are not true Christians unless they follow Jesus first. If scriptures in the Bible conflict with the pure teachings of Jesus, true Christians will always be faithful to Jesus first.
It is also interesting that the Apostle Paul converted to Christianity after Jesus died. Therefore the Apostle Paul was not a student, disciple, or follower of Jesus while Jesus was alive. What the Apostle Paul did was similar to a Jehovah's Witness becoming a Catholic and then leading the Catholic church without receiving any formal education in the Catholic religion. It would be impossible for such a person to embrace all the teachings of the Catholic Church (or even know all the teachings) and discard every single doctrine taught by Jehovah's Witnesses.
1. Matthew 24:45-51. Regarding judgment of our brothers and sisters, Jesus did speak about an unfaithful slave who 'beats' fellow servants of the master. Consider what the Watchtower says about Apostate Jehovah's Witnesses. Also see the Parable of the Faithful/Unfaithful Slave
2. Get The Facts About The Apostle Paul. Is the Apostle Paul God's authority on disfellowshipping?
3. Should true Christians follow Jesus? The apostle Paul? The early Christian congregation? The entire Bible? Edward Gibbon? The Apostle John said, 'Anyone who runs ahead and does not continue in the teaching of Christ does not have God'' 2 John 1: 9 - 11 NIV
1. Watchtower rules and punishments for disobedience. Shepherd the Flock of God
2. Watchtower 2013 convention of Jehovah's Witnesses, speech about Shunning Your Family if they are disfellowshipped.