Remove category from asset group

Hi,

We have some existing and obsolete category codes that are not associated to any asset but are still associated to asset groups. These obsolete categories always seem to be listed in the picklists when using category as a search field for equipment searches.

Would anyone know how I can remove theses categories from the picklist?

My first thought was that I had to dissociate the category codes from the asset groups - I tried this but the application does not allow me to do this. Seems like once you assign a category code to an asset group item, it's there forever.