It’s not as simple as flipping a switch.
Math Captcha was last updated in December 2016, more than a year ago. That is an eternity in WordPress.
I just went to dFactory’s website and tried to sign up for their newsletter but the confirmation page failed and I have yet to receive the confirmation email. Never a good sign.
This plugin used to support Captcha, another free math Captcha plugin. That plugin changed ownership in August 2017 which required not one but two point updates for Strong Testimonials. Then that plugin was all but removed from www.ads-software.com.
This plugin also used to support Advanced noCaptcha reCaptcha but after yet another compatibility issue and the lack of updates and plugin support, I removed that integration too.
In short, to avoid wasting time and headache, I am being selective. In its current state, I will not integrate with Math Captcha. I think 3 integrations offering a variety of Captcha options is sufficient.
Not everything can be free. If you have over a hundred sites, you must be making money with them.
Strong Testimonials is highly extensible. Consider hiring a developer to build a custom integration for you.