http://forums.freebsd.org/showthread.php?t=23127
Unfortunately if you have accidentally added a single drive into your raidz pool at the top-level there is no way to just remove the non redundant disk. Your pool is now dependant on this disk.
If you want your pool to be just raidz vdevs, then you will need to backup your data, destroy your pool, create a new pool, and restore your data.
There is no current way to remove a top-level vdev from a pool.