Disable some of the process functions

I think I have a pretty straightforward question and I'm simply overlooking something.

I subthemed from bootstrap, and now I want to (for example) disable the bootstrap_process_book_navigation function within book-navigation.vars.php.
I tried introducing my own "mytheme_process_book_navigation" function, within my own "book-navigation.vars.php" file (in my subtheme folder), but this does not disable the bootstrap (parent) function. How do I accomplish this without patching?

Thanks in advance!