Hello, Brook. In response to your question, the scripture at 1 Cor 5: 1-13 is often used as a basis for the practice of disfellowshipping. A list of offenses is given by Paul: " a fornicator or a greedy person or an idolater or a reviler or a drunkard" Later on in the sixth chapter Paul lists those practices which would disqualify one for life in God's Kingdom and these are added to the list: " men who lie with men, nor thieves...extortioners"
You might want to take note of verse 9 in 2 John "Everyone that pushes ahead and does not remain in the teaching of the Christ does not have God" The phrase "in the teaching of the Christ" has been interpreted to mean the entire body of teachings. As I noted in another thread, there are practices not specifically mentioned in the Bible that the Society disfellowships on. One such example is gambling. There are some that would argue that someone cannot be DF'd for gambling but let me quote the Watchtower, 1995, 5/15, page 23 here and you judge for yourself: "In 1976 it was clarified that no Witness could be employed in a gambling establishment and remain in the congregation" Now, if merely working in one will not allow one to "remain in the congregation" what do you suppose would happen to someone that gambled there? Since 'gambling' is not mentioned in the Bible, 'greed' has to be listed on the DF form as the reason for disfellowshipping but the real reason the person is removed is for gambling.
Another practice that carries the penalty of disfellowshipping is a blood transfusion. Many witnesses are not sure anymore of what they can and can't do in the matter of blood (and now voting) There are some (brothers and sisters) that have been convinced that it is now a conscience matter. Make no mistake about that, however. Please note jv 183-4 13 "Recognized by Our Conduct: "beginning in 1961 any who ignored the divine requirement, accepted blood transfusions, and manifested an unrepentant attitude were disfellowshipped from the congregations of Jehovah’s Witnesses" There is no Scripture which suggests any sort of congregational action against blood transfusions or gambling other than the way in which "the teaching of the Christ" is interpreted. On the other hand, there are Scriptural prohibitions which do not carry that penalty, i.e. "marry only in the Lord". For this you only get 'marked' for a time.