I hear you say, "reCAPTCHA"? Yes. But what is it?
reCAPTCHA is a free service that protects your website from spam and abuse. reCAPTCHA uses an advanced risk analysis engine and adaptive CAPTCHAs to keep automated software from engaging in abusive activities on your site. It does this while letting your valid users pass through with ease.
reCAPTCHA offers more than just spam protection. Every time our CAPTCHAs are solved, that human effort helps digitize text, annotate images, and build machine learning datasets. This in turn helps preserve books, improve maps, and solve hard AI problems.
And all that for free, from Google. Isn't that nice?
For concrete5, version 6, you can find the "old" reCAPTCHA here: https://www.concrete5.org/marketplace/addons/recap...
For concrete5, version 7, however, there actually is the new reCAPTCHA solution. This got developed by ExchangeCore. Get this Add-On now. Only takes a minute or 2 to install and it will save you lots of headaches.