Signup Secure by Oyabunstyle [used] I guess we use this one for sign ups but new v2 E017 may have its own.
e107 has it's own captcha system. plus there are Visual captcha and Recaptcha plugins available.
But with normal captcha that is problem with users who uses reading device (they are not able to go through because it's image)
I really like your solution, so I updated that plugin.
It's prepared for testing, I let you know as soon as I finish the cleaning.
Of course, I make backup in my old plugin repository.