Well that's part of the problem, stick to less popular subreddits, and you'll get less of the 'riff raff', to quote Basil Fawlty. And when they start getting popular find an even less popular one.
I've been a part of many forums and internet communities and chat rooms, and they always tend to go the same way:
The people who have been around for awhile get increasingly annoyed by the newcomers and their lack of decorum or 'idiotic statements' or whatever, so that segment of the group starts posting in a new more secluded place and use the other place less and less.
Eventually more and more people find out about the 'secluded place' and it becomes more and more cluttered with 'undesirables', (often a few people from the previous group are now part of the old farts, or have become the new old farts), so a chunk of them break off from that community or forum and start posting somewhere even more secluded. And eventually it happens again. And again. And it's turtles all the way down.
I saw the same exact shit happening in real life too with Meetup.com. People stopped posting stuff to meetup groups because they didn't want certain people to come, then there's a smaller group that's secret from the others, then either some people find out about it that shouldn't or one of the people in the secret group starts acting in a way that the other people don't want to deal with them anymore (for example, one time it was because they started dating someone who was a real jerk), so a new, smaller secreter group is formed, and so on...turtles all the way down again.