Hey, this is a situation I'm quite familiar with, both as the PR receiver, and observing PR interactions on other popular github repositories.
How you deal with it is very much down to how you want to engage, and your energy levels too. It can be anxiety inducing too because many of us want to avoid confrontation, and saying 'no' can be confrontational, but you need to remember that the other person is not in the same mindset as you.
Options.
You are perfectly entitled to completely ignore the request. Don't comment any more, just stay silent for ages and ages. The issue can languish for years. If you want you can close it off after a long time. Even then you can close it without comment, or say you don't plan on implementing anything here. You may not like this option as it feels like you're running away, but I'll also say that it's something that happens across many repos for many years, both intentionally and unintentionally.
You can nip the issue (haha... get it?) in the bud. Be straightforward and say that you are not planning on implementing, maintaining and supporting that feature. Your time and energy are limited, and you don't feel it's a feature that you want added to your codebase. Close the issue with the comment. The reaction may be varied but it is important that you stick to this statement and not entertain further commenting, because each new line can sometimes be an 'in' to argue more.
You can take the most pacifist route which is to keep explaining, and refuting, that you don't want to implement the feature. Most people are quite receptive to it and will actually understand, but it's also possible you've encountered someone who is not able to understand your intention. This can be mentally draining as you're speaking to someone of a different mindset, who views you in a certain way, and nothing can convince you otherwise.
But at no point is there any need to insult anyone. Keep in mind it's your codebase, and you will be maintaining the feature going forward. If you do not respect your time, nobody will. Please respect your time OP.