When an unregistered user answers or comments through a Captcha form, there is an empty field "Email if an answer/comment added after mine" and it is checked by default.
This question has an answer that's valid for 1.2 and should also work in 1.3: http://www.question2answer.org/qa/2439/how-do-i-turn-off-email-me-functionality-by-default
It's worth looking into making the changes in a theme file if possible i.e. wherever the checkbox is output in the HTML, override that function in your theme and make sure the box is unchecked.
I thought there would be an option in the Q2A admin by now but there isn't...
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April 20, 2021: Q2A 1.8.6 released!