I would be interested to understand how. Because pep8 validation works on the entire repository and not just on the patchset, the following is what I'm afraid will happen:
1) all code passes pep8 validation
2) someone uploads a patchset that adds a mistake: the merged repository does not pass validation
3) the bot reports the patchset failed validation
4) the change is merged -> the repository no longer passes validation!
And because the repository no longer passes validation, the bot will also report 'failure' on any following patchset!
As such, the repository needs to always pass validation. To make sure the repository always passes validation, no changes should be merged if they fail validation. The easiest way to prevent the changes to be merged is to set the bot to voting.