Comments full of spam despite reCAPTCHA

Hello, I'm having a long lasting issue with spam on the comments sections as you can see here:
https://freedif.org/community-forum-software-how-to-install-vanilla-message-board

the reCAPTCHA is implemented, but it cannot block the spammy comments. Any idea what is the issue and what could be done to improve it?

Also later I need to see how to improve the design of the comments (all packed at the moment)

Start by reproducing the spam problem on the linked comments page and tracing how reCAPTCHA is handled in the plugin. Determine why spam comments are accepted and define completion as effective spam prevention; the separate comment-design request needs its own scope.

Written by the indexing model from the issue text.