Insist that when a student makes a complaint, he or she provides a suggestion to remedy the problem. Have a meeting with the group. Express your concern. Give students a simple, practical exercise. For example, ask a student to present an opinion in a sentence or two and then ask two students to restate the sentence in their own words.
Rep eat for each student.
Or you could ask a student to voice an opinion and then ask another student to disagree with the opinion but without attacking the person. Too many words can overwhelm students. Point out your own mistakes. You will seem more human and students will feel more relaxed. Refrain from personal comments, sarcasm and threats.