Hey, it’s me again ??
I’m totally speculating on commenters replying to themselves. I think the main reason is because it’s simpler and there’s little reason not to. For example, someone may posit a question, and subsequently figure it out for their self and post the answer as a reply to their own comment.
Nearly anything is possible with custom PHP code, but you may not like what’s required. It also depends on your theme. Many themes output comments with wp_list_comments(), but plenty have their own routine. wp_list_comments() uses the Walker class to output levels of comments, so if your own version of the walker is passed to wp_list_comments(), you can manage whether to output the reply button based on the current user ID and the current comment’s author ID. No button if they match.
It wouldn’t really stop someone from replying though, they could go one level up and reply. If no one else had yet replied in the same context, their second reply would appear immediately below their first. At the same level instead of another level down. Depending on your theme, there may be very little difference in the display.
Hiding or suppressing the reply button would not totally prevent replies. Someone could still post a properly formed request to the server and it would be accepted unless you added additional checks to the usual new comment checks. It’s a very unlikely possibility though, so probably not worth adding the extra checks.
Stopping all comments once level 3 is reached also involves the walker, as it is where what level we’re at is determined. Once the walker sees a level 3 comment, it needs to close the current post to new comments, which will suppress reply buttons through out. In this case, it will stop all future comments no matter what.
TBH, the main reason I replied to your other topic which you solved yourself was so it would be taken off the “No Replies” list that a lot of us use. Using backticks or the code button is something we like to see, but it’s rarely worth replying solely for that alone. Anyone posting in the Developing forum has a fair chance of getting a reply from me. In other forums not so much, my kind of coder solutions don’t have much appeal outside of Developing ??
