@niklampe, welcome to our Netlify community site.
At this time, the only way to unlink a repo from a site is to have our support team here at Netlify do this.
The only workaround is to create a “dummy repo” which mostly empty and not used. If you link this repo to the other sites, it will stop the builds from occurring when the original repo is updated.
If you would like us to unlink the repo from the sites, please let us know. Please do so by posting either a link to the site in the Netlify UI or by posting API IDs
Pleas don’t post the keys, as this would compromise the security of your account. The API IDs however cannot be used without the keys are the IDs are safe to post publicly.
We’ll be happy to unlink the repos once we know which sites this should be done for.