I have enabled the "Omit captcha for registered users" option in admin page.
When I open a blog post, SweetCaptcha still visible below the comment form.
I searched inside the codes, and I found that this is just a minor bug in /library/public.php line 65 and 83:
'sweetcaptcha_form_ommit_users' (with double m)
Now the captcha is disabled for logged in users.