Annoying red lines of the spell checker visible when you klick in an exported ibook on a diagram to see it in full-screen even when the function (Edit > Spelling > Check Spelling as You...) is inactivated

Dear Community,

in our ibooks-file the red lines of the spell checker are visible (only) when you klick on a diagram to see it in full-screen. In our ibooks-author-file we deactivated the function "Edit > Spelling > Check Spelling as You..." already also we tried, to "learn" the spell checker the marked words. Still the read lines appear which should'nt be visible. Can you help us? That would be great. Thanks a lot.

Kind regards, Polarlicht