Pre-setting helps save a lot of time because you’re giving a condition about how they should provide feedback and comments. There are also people who are less polite with their expression. In this case, what I usually do is I don’t react very quickly and I try to see if the person persists or not. If there’s only one or two comments, i keep quiet for a while.
If all else fails, I would just bluntly say “If you have very strong opinions of things that you dislike, communicate with me privately in the chat.” Perhaps I could also say, “It would be most beneficial if you send me an email after the webinar so I can address your concern” or I could say “I respectfully am listening to what you’re thinking and I don’t have an opinion right now but perhaps let the moderator know what your concern is about so we can address it later.”
What I usually do is I ask them to practice using the chat. It may be good to encourage them to “Introduce yourself and tell us what you do? What do you like to do? Where are you from? What is your position? What’s your learning objective?” as a warm-up exercises. However, there are audiences that are typically less inclined to participate – cold as ice. Then what you would do is probably do an advance activity, a pre-session assignment or exercise so that they can do it separately and see whether they can submit it during the webinar.
Another way of doing this is to identify one person in the group who simply has a good grasp of responding. So what you do is keep the person as a lead person for people by simply saying “By the way John said this in the chat. Could you read that and tell me what you think of what John said?” By doing that, you’re using an actual and real-life situation from a chat posted by a leading person. At times, by just having a conversation with John in the chat and a few other people rather than all the participants will keep the meeting warmer because there’s an interaction of people.