Substack Chat is available on the web and the Substack app.
If you'd like subscribers to stop replying in a thread, you can turn it off by locking it on the Substack app.
On the app, long press your initial message or select the three dots in the top-right corner. Tap "Turn off replies".
You can always resume replies on a locked thread by tapping "Turn on replies" which will unlock the thread to your subscribers.