If you're using the theme's login/register pages or popup forms you can use one of the following plugins to add a captcha to help prevent spam logins/signups.
- Advanced noCaptcha & invisible Captcha (v2 & v3) – Adds Google captcha to login/registration forms, pages and BuddyPress support
- Google Captcha (reCAPTCHA) by BestWebSoft – Adds Google captcha to login/registration forms and pages (BuddyPress support requires pro version)