These captcha might be all sort of mechanized solution that trolls are well aware. Perhaps there are humans behind the registration, just to get a new place to put their ads.
I was rather thinking about a formula that is propose a picture or a sequence of them and ask the description via email. Definitely a human authorization is required from both party. So the real registration, might go down for a couple per day.

Spiders like Yandex, Google and Bing need to be given the way, because will help researches for some particular user case. Unless the internal search engine is good enough to give an expected result. Anyway, blocking these spiders may reduce the site popularity.