Forums / Cotonti / General / Russian spam?

That's a definite overdose: most users will leave your site forever rather than trying to figure out what

in welcher Sprache stelle ich dir diese Frrage? (Klingonisch ist es nicht):

is.
I would assume that ANY captcha is a 99% protection for a low-to-medium ranking site which 99% of all Cotonti sites are. If a spammer is trying to place ads at your site, "ban by email" is good for nothing, since switching to a different public mailserver is a one-minute job.
As regards "top-10" megasites, an overcomplicated captcha might inevitably cause loss of audience in comments (don't make me think!). Such websites require "manual" moderation anyway, so once again this is not the case to play quiz with your unauthenticated visitors.