Clear out all Data in a Field - without deleting the field itself

Hello,

I feel as though I'm overlooking something very simple here.

Is there any way to clear out all data in a specific field, without deleting the field itself?  Or even better - clear out all data in a specific field - if certain parameters are met?

Example:
I'm keeping track of the status of images in another system ( available / unavailable, etc ).  If an image is unpublished, it is generally moved out of the library, and that status in QB should be null.  Unfortunately, the library can only report on what's in it - not what's NOT in it.  So how do we wipe out all of the other data? 

I do not want to do this every week - most of the time, just loading new published dates is OK, but once a month or so - I'd like to wipe the "Publish Status" and start from scratch.  A full report every week is too time consuming and the difference is more efficient.

Any ideas?