Only spells? Or also skills?
I could possibly envision a sage or magic user with a lab/study where the Champ could create a Tome, possibly writing down 1 to 3 spells that clears them from their repertoire and they purchase the tome based on the skill(s). But probably have to have it so they'll only buy it if the spell is one the Champ hasn't scribed before (to keep player from running out and Mirror Stancing something and just doing it over and over.) Purely as an in game method.
I mean, other methods include drinking until you lose your memories or something (a typical night with Antonina?). Otherwise maybe an option to click Hide that renders the selected skills invisible until you check or uncheck a Show Hidden box, so you still have them, but don't have to scroll past all of them (though the Search option is pretty good when you at least know part of the name).