Preventing Spam in phpBB3 - this topic not spent requirements in my opinion.
Most of preventing spam not working as solution (not technical problem but in practice) due advanced spam solution, eg. AI/ML etc. in test, and writing posts..
I have many fake user, that still registering, and fill user list. No spam post coz need admin manual confirmation, but it's problem to control who is real and who not.
By this looking some Custom Registration Fields where can be:
1. simple pattern with some advanced knowledge question from panel
2. advanced pattern with some advanced knowledge question from panel with other function that can be created by php, eg. as string contains from sentence or select signs from sentence, image or hidden image or from 3 images.
3. more CAPCHTA images (Eg. 2) to operate in point 1,2 to build some sense words with colors.
4. mixing languages, what can help in some special branches where default bot not have knowledge
5. give some sounds to play, to repeat sense or do some question/task to answer (have sense and good limitation for languages and special branches area)