One of the first tasks that I had done when I started my programming career was creating contact forms for websites. That was the time when I heard about captcha which are used to prevent spam form submits. We were using php scripts for generating captcha and later we used Google recaptcha.
I shifted many companies after the first one and I never experimented on any other solution.
But in one of my personal side project that I created I was receiving many spam form submits through the Sign-up page. So I done an experiment which went successful till now. I will not say this is an ideal solution. But in my case it worked.
I tested this in one of my site called Cherukatha.com(https://cherukatha.com/) for the signup page.
What do you think. I am open to your thoughts, ideas and suggestions. :-)