How to turn off Discussion Replies
If I understand, you would like the students to be able to post an initial response on a discussion thread and like the initial responses, but not reply to any other posts?
I don't think this functionality exists in the native Canvas discussion platform. The only customization similar is to disable threaded replies, but it won't limit students to only one initial post. For that you may need a third party program that would allow you to moderate posts. Off the top of my head, you can use Padlet for that by allowing students to contribute to a board but you would moderate/approve the contributions.
Similar to what @DrNufer said, the discussions are specifically for students to post comments. You can disable replies, but to disable all posting of students would need to be changed at the admin level. If you want the ability to post yourself only, and have students see or like - you could consider posting announcements. You can comment on your own announcements below and students could also "allow liking" but disable them from commenting on the announcement. Besides that, you'd want to use something external i.e. linking to a Google Doc, etc. Hopefully this helps to clarify!