Captcha for Reply
Is there any option to enabling captcha for user replies.
I'm sorry, but there isn't. You can enable it on login form, so the user will be verified on login process. If a user is not robot on login process that he/she is not robot on creating a new topic.
I'd recommend check the #3 and #4 points of the spam protection topic: https://wpforo.com/community/faq/how-to-stop-spam/