How to remove pre-installed Flash Player in Windows 8?

Officially this is not supported.

Make a complete Windows Image (recImgexe) first and download the tool win6x_registry_tweak. Now open a cmd prompt with admin rights, in the folder where you stored the Exe, and run this command

install_wim_tweak.exe /o /c Adobe-Flash-For-Windows-Package /r 

This kills the Flash package. After a reboot it is gone.