My Mac book has been remotely hacked
Honestly if it was me, I would make sure that WiFI WAS OFF (Like you said it was.) and then wipe the system completely of any and all data.

Of course this is assuming that you have a backup of your files previous to the hacking. If not, well then it gets complicated figuring out what files are safe to use.

So to wipe the Mac, boot it up while holding CommandR until you see the white Apple logo on a black screen.

When you get to the macOS Utilities screen, select disk Utility from the Utilities menu. Select your hard drive on the left and click on Format.

This will erase EVERYTHING on your drive

Once that is done quit out of Disk Utility, connect to your WiFi, and proceed with macOS install from the main menu.

This will likely install the version of macOS that your Mac shipped with. You can upgrade it for free once you have done this installation and restored your files from backup.