I understand that you had to restore the Notebook to factory settings because you were unable to change the previous owners sign on information.
You had asked why there is not an easier way to delete an old user. I don't have an exact answer to that question, but what I can offer you is links to Microsoft so that you can ask about sign on information and if there is an easier way. You can reach out to them using their Microsoft Community Forums just like you had done here.
Here are a couple of documents you can take a look at that may help answer your question: