Out of the box it is not possible to noindex
Threads.
Though it could be done manually (via modifications to either PAGE_CONTAINER or smth. like that), it would make more sense to use an Add-on for this, this way threads could also be excluded form the sitemap.
We've implemented this (and other features like noindex for a whole forum, meta descriptions per node, <title>per node, <h1> per node, extended description text for nodes, meta description per thread, etc.) in a custom SEO Add-on.