If you place a Photoshop .psd in Illustrator, any spot color channels will automatically create spot color swatches. Helpful and cool. If you change any spot color names in the .psd, updating the links will give you extra spot colors in Illustrator. It’s usually easy to use the color palette to select and delete all unused colors. Sometimes, one gets “stuck” and the trash can is grayed out.
I finally figured out how to unlock and delete those spot colors.
Select all the placed .psds that had used that color. Choose “relink” in the Links palette. You don’t have to delete and replace, just relink each one.
You should now be able to select and delete that unused swatch.