Edit: Thanks for your kind comments on disabling contributions. I'm aware of the possibility, this is just a meme.
Contributions in open source projects are great, needed and make the world a better place after all. Therefore they should be declined politely, optionally by giving a reason.
I hereby inform you that I will no longer accept contributions to this post /s.
While I do agree GitHub is a place where people collaborate, it's also a pretty handy place to store stuff without having to host your own. If the project doesn't invite people to contribute, don't expect a polite response forever. It's like stopping your neighbor on the street corner to tell them they should paint their house white for the thermal benefits, yeah people collaborate on the street, yeah you are right, it's their house though.
Github provides ways to both display code just as is (archive mode) and to also disable issues and auto-close all PRs with a message.
Is archive mode the appropriate tool for this? I would assume that's for when a repo is no longer receiving any updates from anyone, not just when the owner doesn't want others contributing.