At the moment, the only way to delete a linked library is with an outliner operation, which can be executed only if the library is selected in the outliner. There is no way to delete a linked library with Python.

I've created a function to delete linked libraries with Python. I couldn't name it delete so I named it remove.

Its behaviour is similar to the reload operation. For example, supposing to have a linked library C:/library.blend, to delete it with python you would need to use the following code:

bpy.data.libraries['C:/library.blend'].remove()