You can stop building your project on CircleCI at any time by navigating to the project settings page and clicking the "Stop Building" button.
In the event you are for some reason unable to stop building, you can always manually stop building by removing access to the repository from GitHub.
1. Start by accessing your GitHub repository settings. You can visit this directly by navigating to https://github.com/Org/Repo or clicking the "settings" button on the repo page.
2. From the settings page, on the left side in the vertical navigation, we see the two pages we are interested in, "Webhooks" and "Deploy keys".
3. From the Webhooks page, locate any Webhooks that post to CircleCI and delete them.
4. Lastly, navigate to the "Deploy keys" page and delete the key added by CircleCI
This will stop the project from building on CircleCI in the event you need to manually remove access.