If you draw a firm boundary with that contributor, and they continue to push, ban them. "This doesn't meet the standards of our project for reason xyz. Please

If you draw a firm boundary with that contributor, and they continue to push, ban them. "This doesn't meet the standards of our project for reason xyz. Please refrain from submitting further PRs that do not adhere to our contribution guidelines outlined in CONTRIBUTING.md." If they continue, ban them.