As far as I know there is no way to delete a source using the online interface. To keep from inadvertently using the source you could change its name to “Don’t use this source” or something similar. Unfortunately the online tree interface doesn’t have a direct source or citation management interface so this gets a bit complicated.
Pick any individual having a timeline event with a source citation. Click on “Facts and Sources” tab, then click on “Source Citations” tab. It would be easiest to pick an individual with the source citation you no longer want to use but not required.
You’ll see each Source Citation listed. Pick one, the one you no loner want to use would be nice. Ignore the “View image” and “View record” buttons, instead look for “n Citation(s) provides evidence for the following:” followed by Detail. For example 1 Citation provides evidence for the following: Name, Birth. Detail .... Click on the “Detail”.
You will then see a page titled “Source Citation”. Click on “Edit citation” (over to the right). Then you’ll see a page with “1. Source”. Using the selection box arrow find the source you no longer want to use, click “edit this source” (just below the selection box). In the edit source panel change the Source Title and click “Save Source”.
There might be (probably is ..) a more straight forward way to accomplish the above but this is my best shot.