Thanks, all of you.
I tried putting in just one attribute (Fill: red) as you suggested, David, and that worked in the sense that it found the very first of all the red diamonds. Step one completed.
And thank you, Jongware, I had overlooked that Style Options section, thinking it was for something other than defined object styles — silly me!
But I am still confused about a few things.
Please, dleather, how do I create such a “nothing” style? I tried the suggestion of defining a new style (called “Nothing”, of course) with just one characteristic — I set opacity to zero. Then I ran the Find/Change dialog box and it found the first red diamond OK, but when I clicked “Change”, it stripped the diamond of all stroke and fill and left the (now empty) frame in the document. It would no longer print, of course, but it was certainly still there. Had I chosen “Change All”, I would still have had scores of empty frames in the document — not a desirable outcome.
My guess is that having any other single attribute would achieve the same result. Jongware, I don’t think it is possible to define an object style to include Non-Printing. It doesn’t seem to be an option anywhere in the New Object Style dialog box — but then, I have been known to overlook the obvious in the past! Even so, it would not be an automated means of deleting all these no-longer-wanted objects, simply of making them inert.
I will certainly go for the separate layer option next time around!