When I was working as a team lead, I rejected any PR that was not in line with the rules. I stopped the review immediately I saw the rule breaking line(s).
One time, a manager asked me why I rejected a PR at the end of the sprint, before the demo, and I said, as you wrote above, that it's my responsibility and if he wants to overrule me, he should do that officially by assuming responsibility and escalating to our boss.
Make sure the rules are clear and then stop trying to appease.
edit: I wanted to make it clear that the rules were broken only when a new dev joined the team, after a rejection or two, PRs were extremely rare instantly rejected.
One time, a manager asked me why I rejected a PR at the end of the sprint, before the demo, and I said, as you wrote above, that it's my responsibility and if he wants to overrule me, he should do that officially by assuming responsibility and escalating to our boss.
Make sure the rules are clear and then stop trying to appease.
edit: I wanted to make it clear that the rules were broken only when a new dev joined the team, after a rejection or two, PRs were extremely rare instantly rejected.