Board Thread:General Discussion/@comment-35075659-20180324003300/@comment-32182236-20180324004153

There's no official way to do this. You could delete the file "system_information_963", but Steam just restores that file back... So, unless you're playing offline for the rest of the time you play the game.. There's not a way to officially remove the flag.

However.. There HAS been a program created specifically to remove the effects, called "Undertale Genocide Removal".