Hi @ctaggart,
I guess you mean build only a single branch automatically.
For that please see: http://devcenter.bitrise.io/webhooks/trigger-map/#how-to-build-only-a-single-branch
In short, in the Workflow Editor you can set what should trigger a build (Trigger tab) and which workflow should be used. Simply remove the Pull Request one if you don’t want that and change the Push one from the default * to the one you want to build eg master. You can add multiple patterns too, so if you’d want to add a second one you can do that there, and you can use * as a wildcard, eg feature/* will build all branches which start with feature/, eg feature/xyx.
If you’d have any questions just let us know! 