Have you already used Master Categories? If not, it can be a very helpful feature of activeCollab that enables you to create default categories which can then be used across all your newly created projects.
To set-up this handy feature simply go to the Admin > Master Categories and create categories you wish. These will then be available for all Discussions, Files, Pages and Tickets across all new projects in activeCollab.
Please note that fresh categories will be applied only to the newly created Projects, and that they will not be changed or added to projects that are already running.
If you don't need one of the default categories, for example a Tickets section in one of your Projects, you can delete it simply by going to the Manage Category tab and clicking the X mark next to the category name.