How to disable functions in parent wordpress theme?

I'm making a child theme to the Twentyfifteen theme in wordpress. The theme has a function in it's functions.php that I wan't to remove:

set_post_thumbnail_size( 825, 510, true );

I can remove it from the parents functions.php but I don't want that because I wan't the parent to be untouched. Does anyone know how to do this.

(I wan't to remove it, not change it)