How to tear down a geom mirror?

[Peter Steele]

>gmirror and various other geom modules store their metadata on the last 
>sector(s) of the drive, so you need to wipe that too. 

In our case the systems we are using aren't mirroring the whole drive, just certain slices. Some systems have a single slice mirrored (plus an unmirrored slice), and others have two slices mirrored (plus a third unmirrored slice). I need a way to destroy the existing mirrors, without doing a gmirror load, and ultimately without making any assumptions about the number or condition of mirrored slices on the drives I am about to install a new OS onto.