Hi,
Thank you for your question. With regard to captchas and accessibility, there are several fundamental problems, which you can lea more about by reading the information here. https://blog.sinabahram.com/explaining-the-fundamental-accessibility-challenge-of-captcha/
As an alteative to a captcha, you should consider using something like a logic or math-based question that a user must answer in order to submit the form. WordPress plugins for this include:
Human Captcha: https://wordpress.org/plugins/humancaptcha/
Math Captcha: https://wordpress.org/plugins/wp-math-captcha/
These are for securing your Idea form, registration form, and login form. If these or similar from the WordPress plugins repository don't help, please feel free to reply to this post with more information so we can narrow down some choices.
Thanks,
Amanda