I did have an interesting talk with lainons about barriers of entry. Some imageboards have anime captchas (identify all the pictures of [x]), although I was able to find a booru and guess right first time. Another (fatchan??) has a chess-based captcha to weed out retards.
Then there's ones like 4kev where image posting requires an account.
Some communities also have strong cultures where spotting newfags is easy, even if anonymous. Apparently Ponychan since 2011 intentionally banned anything resembling 4chan culture to try and breed a unique one, and I'm sure they weren't alone in this.
The issue for /leftypol/ is that we do want to allow newbies, and debatably, potential converts. A relatively low barrier of entry is part of the site's MO, and our culture is pretty horrible at weeding out bad posters (bait biting). I don't see it changing, and it would be somewhat futile as we're q boogeyman for the largest imageboard in existence getting mentioned about a hundred times a day even without happenings.