I think the simplest solution (if WK wants to implement this, which I doubt they do), is to let you “ignore” certain kanji.
You can’t ignore radicals (because they are used to learn other kanji in the future), and you can’t ignore vocabs (because one kanji can have several readings and meanings, many of which are reinforced in the vocabs).
But if you ignore a kanji, it would stay (technically speaking) unlearned. You don’t have to do the kanji lesson, but you unlock the vocabs for that kanji. Although you have not learned it on WK, it would behave as if it were burned.
That’s the closest thing I could come up with a solution for people who already know lots of kanji before WK.