One of my laments has been that I can't (easily) flag a repository in Github as archived.
As of today, you can!
Github just announced the ability to archive repositories, and yes, it can be undone.
To archive a repository, go to your Repository Settings Page, scroll down to the Danger Zone and click Archive this repository. It will pop up with a screen to warn you that this is a serious change, and while it can be undone, it will ask you to type in the repository slug to confirm.
Once you've closed the repository, you'll see it has a new banner at the top, announcing "This repository has been archived by the owner. It is now read-only."
The repository will also have a little 'Archived" badge in the regular view.