I think these can be helpful...
Draw a rectangle box with that color. and go to Select>Object>Same Fill color or Select>Object>Same Stroke Color, then you can find the similar color objects.so that you can easily remove.
Method2: (If you have acrobat professional)
Create a PDF file and open that PDF file Acrobat Professional. go to Advanced>Print Production>Overprint Preview, there keep only that color. then it shows where you have used that color. so that you can easily remove
hope will helpful...
If you remove the un-used Graphic Styles, Symbols, Patterns, Gradients, Brushes, Character and Paragraph Styles, and then attempt removing this color, you may be successful. Colors used by such un-used elements, actually are used colors.
I hope this helps!
Created the box with the color, unlocked all layers, selected "same". I found a long forgotten and probably mistaken empty, zero dimension, text block that must have been styled in the deprecated color. Probably I had the loaded text tool and "clicked", then forgot I clicked....maybe months ago???
All gone now.