This question already has an answer here:
I have a MacBook which OS was upgraded from Snow Leopard 10.6.3 to 10.6.8 and then to El Capitan 10.11.
I want to completely reinstall El Capitan, to "factory state", in order to really "clean" the Mac. (However, I don't need to prevent data recovery.)
I started the MacBook holding down the "Alt" key and selected the partition for reinstalling, logged to Apple and the download of El Capitan was progressing.
Then, I moved away from the Mac for about two hours. When I came back, the user account is unchanged, with previously installed apps. Seems like the installer decided in my absence to keep the user files.
Can I resintall El Capitan to "factory state" without wiping the whole drive? Or should I create a new user and then delete existing one?