Take a look under Settings Control Panels Categories
there seem to be a number of references to about this, so you can experiment with changing the configurations.
Keep in mind, Tiki can be used without Categories (I never use them as they add an unnecessary layer of complication on top of an already complicated 'User Permissions' and 'Group Permissions' set of configurations (more than enough granular control with these two, IMO)
Reassuring to know that Tiki can be used without Categories.
I found the easiest solution was to change the Theme