Spam subcriptions

Hello!
Since a few weeks, I get soo many spam subscriptions. I’m glad they are not beeing activated because of missing confirmation of the opt-in mail. If I add a Captcha in Cleverreach sign-up form and press “Load Form Attributes” in my WP-Backend, there is no captcha-field for the form builder.

Which is the best way to solve this?

Thank you,
Mike