Doesn't look like there's any way to do it in the UI.
But, you can edit the /usr/share/kde4/services/plasma-runner-bookmarks.desktop file.
kdesudo kate /usr/share/kde4/services/plasma-runner-bookmarks.desktop
Find this line:
and change it to:
You'll have to restart plasma for it to take effect:
I'm not sure if this change will persist when KDE is updated, though. My recommendation would be to instead avoid all this trouble by just using the lancelot application launcher (package name is
plasma-widget-lancelot). I prefer it to kickoff, and it has a UI that allows you to disable the bookmarks runner for the search bar.