KEPUB automatic hyphenation is broken in English too, it is not just a problem with Dutch.
From what I read in the
KoboTouchExtended driver thread the driver has an option to turn on hyphenation when it creates the kepub, so you will need to make sure that option is turned off.
But the publisher might also have turned hyphenation on, so to turn it off you could add something like
body,div,p { -webkit-hyphens: none !important; } to the book's stylesheet (which you could do automatically for all books by adding it to kobo_extra.css and enabling the "Modify CSS" option in the driver.)