From what I gathered, there may be two different things you are trying to do:
Remove the category from the rule To do this, open/edit the rule and click the 'x' next to the name of the category.
Remove the category from the drop-down list (i.e. from the entire property) It's on the roadmap to add this functionality, but it's not yet there. If you'd like one or more categories removed from the drop-down in the meantime, you can send us a private message with your company name, property name, and the name(s) of the category/categories you want removed, and we can take care of it on the backend.