News reader has a traditional kill thread feature. Similar feature would be nice for mail folders, so that threads on mailing lists could be killed. Discussion lists, after all, are very similar to news (on the usage level) and it is natural for a user to expect similar handling (I do expect it :) ). I guess there will be somewhat more hassle with identifying threads on mailing lists, but it should be doable.
This looks like a dupe of bug 179033.
*** This bug has been marked as a duplicate of 179033 ***