Follow this thread:
In short, with Thunderbird, Tools > Message Filters > Edit > Filter
Rules > Add item (use "+" after last filtered item in list), Choose the
filtered item (eg. Subject, From, Body...) scroll down to Customize,
click on it.
Customize Headers pops up. Type "Newsgroups" in the textbox. Hit OK.
Exit Customize Headers, back to Filter Rules. For the new entry, add a
filtered item, select Newsgroups from list, enter part or all of group
(misc or misc.survivalism, for example). Hit OK. You might have to dump
out of Tbird for the new filter to work.
Now if only folks would stop answering cross-posts in this groups, you
wouldn't see any of it. But it IS more focused on metalworking.
My filtered terms under Newsgroups: alt, misc, and talk.