Yeah, where's the setting for that? Hmmm.
Probably, you messed up something with Google Now. Maybe you could go into Settings | Manage Apps, and clear Google Now Cache?
You might try downloading "Google Now Swype Disabler". Then I think you can reset the possible stuff to default (or choose another app to launch in it's place). If you successfully reset stuff back to defaults, then I guess you could uninstall the disabler program.