Together with @meetdilip's assistance and willingness to provide his forum data for further analysis of the spam, I have now blocked some of the spam IP ranges present on his forum. If your forum experiences large amounts of spam, then please email me at email@example.com with spam usernames and your forum URL, and I will look into it & block them.
I am aware that these actors have multiple IPs that they can switch at anytime, so please don't expect this to be a 100 % effective counter-measure. It's rather a provisional measure until we find a better and more effective way to fight spam. I am thinking of automatically fetching spam IP lists from public sources and updating our filters accordingly