Can anyone help with a hacked and hijacked MacBook Air and iPhone 6splus?

Both devices are hacked and hijacked. Have been back to the Apple store multiple times, and I have reinstalled the OS more times than I care to remember. This ******* just sits in my devices and changes permissions, deletes files, and runabout it as a bit torrent server. Multiple connections that I can’t do a **** thing about. Any guidance would be welcome. Also, this is my first Mac. Good thing I bought it for the security reputation of Apple.