Sorry for the delayed response. Been busy this week with a new release and some other tidying up.
Definitely agree about spammers. We hate them as well. On a side note, we use CEDIT Registration Validator, which has been invaluable for catching most 'baddies' before they register.
Anyways, regarding your problem, I'm not sure why the captcha would interfere with JFBConnect. I haven't tested yet, but does the Login With Facebook button not work on any pages, or is it just the K2 page? Is there a specific way I should test? I don't want to login with Facebook just to have an account created on a page that works.. since then I won't be able to test on a 'bad' page until you delete my account.
Let me know, and we'll help you get to the bottom of it.
Thanks,
Alex