Ususally, I just try to filter out off-topic newsgroup posts by keywords in the Subject or Body fields. Unfortunately, some people are enthused about discussing someone who happens to have a surname similar to certain types of plants. (Note that I decline to discuss any intelligence or morality similarities.)
There are thousands of separate newsgroups, carefully sorted into different subjects.
When I read rec.gardens and rec.gardens.edible I am looking for discussion on (drum roll please) gardening.
If I wanted to read discussion and debates about which criminals are running which gubmint in which country, I would go to newsgroups where those topics were actually relevant.
Perhaps certain people could start placing some warning keyword in the Subject line of their non-gardening posts. Then, folks interested in gardening could filter out the non-interesting debates.
Thank you.