Nothing I do seems to have any effect.
You can disable them in SKS5; works for me
I haven't been able to get them disabled in SKS6 either. And there is no support for Fonts for Classic Widgets.
Personally, I'm looking forward to a SKS6 that does everything SKS5 can do and much more.
Until then, I'll do the majority of work in SKS6 and I'll keep SKS5 for those bits SKS6 won't yet do.
Yes, I know SKS5 can 'break' a skin made in SKS6 ... already had that happen ... if you haven't seen the UIS1 error when trying to open a skin in SKS5 that was made in SKS6, keep using both and you most likely will.
But not anyway around it that I can see, for now ...