I know someone working on a smaller open source who has same thing. They have considered just blocking all PRs outside known contributors because AI spam even on their tiny open source project is too much.

At work, I've gotten into fights about PR approvals. If they are beyond us humans to review, screw it, remove the approver requirement and if CI passes, merge it.

Just an idea which I haven't personally tried: AI agents understand technical limitations, such as CI failures. Maybe make a CI job which checks that a PR has a reasonable size, and auto-reject with a polite message if it's not? Something like, "This PR size exceeds the limit of N lines that we accept for review; if you implement a big feature please consider splitting it in several smaller PRs." There are chances that it won't help, but it might!

A simple solution might be to use a git hook that asks for confirmation if it is too big, with a suggestion to ask the user to have the agent split it up.