Disable idea categories Merged

I would like to be able to disable idea categories so that I can keep them from being used anymore without losing the category tags on existing records.  Today, my only option is to delete them, which also removes the categorization from existing records.

Here's my use case:  I'm re-purposing the idea portal related to a product. I've changed the flag of the existing ideas so that they are no longer visible in the portal.  The old categories don't apply to the new purpose but they show up as options for new ideas.  My only option is to delete them but if I do, they will be removed from the legacy ideas as well.