Some of my sites keep being attacked by spambots posting this nonsense. I leaned heavily on the Aquia service Mollom, which was a great module, but that service unexplainably stopped in april 2018.
So, what to use on Drupal 8 sites to stop this useless salesy bantering and pointless digital screaming on our sites:
The first module you can deploy is captcha. This is the simple Stop and frisk module to see if you are human.
ImageCaptcha, is the Stop and frisk and ask some more questions version of the above. You have to interpret an image before you can send in a form. (I always find this one very annoying.)
This is the user friendly update of the above two modules, it simply asks you to check a box titled: I am not a robot.
As long as the artificial intelligence is on the current level this module will suffice.
This is a module that requires no user interaction at all, so it is my first choice to use. Depending on the user interaction times it decides whether you are a human or a bot. Furthermore it does a hide and seek action with fields that the bot thinks it needs to answer but the humans do not see. This module worked great, but of course the bot priogrammers are not stupid so they are finding ways to fool this one......
This one is not yet available for Drupal 8, but it seems to provide the same functionality as Mollom did.
This one is rather affordable for drupal: 5 euros per month and seems to provide the same functionality as Mollom did, but also seems to use more advanced technologies... (Or they know how to present their products in a sexy way). I did not try this one out yet, and update here as soon as I did...