Safari hijacked by Safe Finder. Change homepage preference greyed out
Deleting your Preferences properties file should do it. 


Close Safari completely. 
Open a Finder window and go to your Home folder (the one with your Username). 
Go to the View menu, and select "Show View Options". (Alternatively, you can hit Command+J.)
At the very bottom of the options window, select (check the box) "Show Library Folder".
Now browse through these folders:
Library/Containers/com.apple.Safari/Data/Library/Preferences
Once you drill down to Preferences, you'll see a file named com.apple.Safari.plist.  That's your preferences file.  Delete it. 
Re-open Safari. Your preferences should all now be back to default.  You'll have to set things back the way you had them before, but you'll be rid of the messed-up settings.


That should do it!