Currently you have to make a support request in order to unlink a project from a repo, which seems to be the only way to prevent automatic building. Given that a bunch of people have had this seem need, why not make it a feature?
The reason I need it is because I’m integrating with another CI tool, and I want that tool to tell Netlify when to build, for which I’m using the Netlify CLI. Sadly there is no publish command available in the CLI (another feature request which would solve my problem), so the only way around this problem I can see is to unlink the project and then use the CLI to build (with auto publish turned on) instead of the git hook triggering the build every time. Currently there always has to be human intervention through the Netlify frontend, which is not an option for me.