At first, my brain parsed "spam" as in ye olde email spam. It took a moment for it to sink in that you meant code spam, instead.
But now that the two concepts are linked in my little pea brain: I kind of want to see how a system like SpamAssassin would work when applied to pull requests like it has been applied to email.
It can use reputation, real-time blacklists, triggers for form, and et cetera, with weighted scores for every aspect that are ultimately factored into one final score.
If final score is passing, the PR is presented for a human to review like a PR was (say) 5 years ago. If the final score fails, it goes into the circular file where it will probably die.