Thinking about that problem, it seems to me that the only feasible way for Kagi would be to integrate with other sources of truth, like Vlad said: blacklists, or something similar (but imho common blacklist will be fine, at least in the beginning).
About the "selective domain block filter" as suggested by mesaoptimizer, I've to admit that I'm quite biased: I'm from Italy, and .it is my "national" domain, and my results are usually full of ".it" domains.
So, I'm quite sure that no italian Kagi user will never filter by ".it" domain trying to catch the spam; and the same will be for Kagi french user and ".fr" domains, or australian users and ".com.au", etc...
So my proposal will be: integrate blacklists, if it's possible for Kagi architecture.