I don't agree. If the PR you're reviewing is expected to be merged into a development branch, you can either:
1. Merge without comment. Unless the contributor is closely following development, they won't even notice what you do with the contribution between your development and release branches.
2. Merge with a comment that you'll be cleaning it up before release, and if you want the person to continue contributing explain what you'll be doing.
Differentiating stewardship from producing solutions can go a long way toward eliminating debate over minutiae.
If there's still tense back and forth, it's probably over approach, understanding of the problem, or awareness of consequences. In none of those cases should the PR be merged. If you want to engage the contribution, explain what the contributor missed and close the PR. If you don't think the contribution is going to be productive, just close it.