You have a number of different options depending upon how you want to operate (GUI or CLI) and also to what extent you want to operate with the information.
I'll make a number of assumptions and you tell me when I'm horribly wrong!
First scenario: I have uninitialized snapmirrors and I want to get rid of them.
GUI: Go in to Filer View, find the entries in the Snapmirror section and choose delete.
CLI: rdfile /etc/snapmirror.conf - Modify and remove the line from the un-init'd snapmirror - wrfile with the results.
Alternate: using snapmirror release
systemA> snapmirror release vol/vol1/qtree2 systemB:/vol/vol2/qtree5
Now, I would be ignorant of the reality of your situation if I were to stop there.
Here is what happens in most scenarios when following through with the steps I mentioned above:
Someone goes to the destination, releases, deletes or otherwise using CLI or GUI, and they go about their business with no more snapmirrors being sent.
A few days (or weeks, or months depending upon block changes at source) goes by, and all of a sudden they're running out of snap reserve at the source.
If/when you break an established snapmirror relationship, it is pertinent to go into the source and ensure that any leftover snapshots are cleaned up.
An example may be like this showing up in a 'snap list'
0% ( 0%) 0% ( 0%) May 15 15:45 FILER(00000000)_filer_mirror.8160 (snapmirror)
So being that it is now December, and you have a snapshot out there from May, this could be a problem!
That being the case, I'd suggest cleaning it up!
There are ofcourse other scenarios and situations in which I'd deal with your question and problem, but I'd start there.
Did I touch on your question or miss the ball completely?