And what makes you think catpcha is any use against bots these days? It isn't. In the war against spambots captcha is obsolete. Just about any bot can read any captcha a human can manage to read, and for the bots it isn't annoying. I have deliberately left the captcha turned down to a low level on this site and have even thought seriously about removing it entirely.
not really
ocr has still a limited success rate against modern forms of captcha
as an alternative, add a huge delay before authentication (like 20 seconds?) when logging in
that will slow down the number of attempts per hour and make any dictionary attack unfeasible