luceos Thanks for the idea. Is there a thread about Akismet in the Extensions tag? I couldn't find it (even with search).
I did find the Git repo and tried to install with
composer require flarum/akismet which worked.
In order for it to work, do I need to pay for a subscribtion on akismet.com? It's fine if I do, I just want to verify that is what you intended me to try.