Categories should be filtered per module. Two clients have being requesting this feature and i am sure there is more people experiencing this limitation.
The category table is locked down, we need a new field called module [enum - to all modules (request, change, incident, release, and etc)] that you can specify when adding a new category record. this will allow us to filter those categories per specified module.
if the categories specific to each module are all grouped under a specific subset of parents, you could add some rendering rules (Define suggested values based on filter) and enumerate the parents.