The default/built-in setting to build PR’s or not is “all configured branches”.
If you want to modify that behavior, there are a few ways you can get there, such as posting your commit webhooks from your git provider to an intermediary service like zapier where you can filter them and only send the ones you want us to build onwards.
As far as “no additional services needed” options, and probably the one I’d recommend, you can use the build.ignore script to run git commands to see details about the commit and choose to return true on the ones you don’t want to build: