Tell them no. Thats what I do. I have rejected multiple PRs that were too large and lacked proper design or alignment upfront. With code being so cheap, rejecting it should be just as cheaper. Set cultural standards that devs need to review their code before asking for reviews. Etc etc
I don't think it has the effect you think it has. No-one takes a rejection personally anymore since it's so easy to just tell an AI to fix the comments. So a rejection does not make them rethink like it would have back in the day.