Threads/Sub-Threads

[oly51]

Members on my site will start a thread on a project they are working on and update it as they go. Other members add comments that go off topic and often the core of the project gets buried.

What would be the best way to keep the comments separated from the main thread? Is there a way to have a thread open only to the thread starter and all comments get added to a sub-thread? Is there a mod available to do this? (It's 6:00am and I have not had time to do a deep search on here)

How do others handle this on their sites?

Thanks