I think that besides using the text replacement feature (which I consider being a workaround, not a real solution - having to create a replacement for all words that I misspelled can become annoying) there is no other solution than to try and re-train it over and over again, until it 'forgets' the misspelled word (hopefully).
I ran into the same issue a couple of times, and did not find a real solution.
This is one of the things that I dislike about Apple, they think they know better what users should want. And if user(s) do not get along with how Apple thought it should be, there is often no option for customization.
I mean, how difficult would it be to allow for editing a user's personal word correction list? Even my stone age Blackberries could do that, if I remember correctly.