Uninstall SuperSu (or Superuser or whatever you're using).
su
rm /system/bin/su (or /system/xbin/su)
If busybox is installed (in /system/bin or /system/xbin) rm it too.
That won't change the status of the OS (it'll still show that it's been rooted), but root apps will no longer run, which is what unrooting is supposed to accomplish.