Want to be able to delete a pipeline we're not using
If you want to stop Pipelines from running, you can disable Pipelines from your repository's Settings > Pipelines > Settings.
If you want to delete an already-run pipeline from the UI, that's currently not possible. If this is what you're referring to, can you please help me to understand why you would like to do this?
I'm going to resolve this issue. Please feel free to re-open this if you feel like your question has not been answered.