You might be able to do a factory reset, which would wipe everything.
You could format the hard drives.
You could have my computer technician come and make sure everything will be done exactly how you want. If you or anyone else is interested, his name is Choke. 089 212 7696. LINE:Sing168. He speaks English, highly reliable and fully competent, charges very reasonable fees, comes to your home, and comes at the agreed time. I have been using him for years and would not consider anyone else. And . . . he is quite good looking, but unfortunately not gay. Oh well, can't have everything.