well an assigned album is a personal album which means you allow users to create personal albums and also you allow users to assign albums to categories so there really is not an issue here as you allow this.
The point is you as admin would need to investigate this and modify accordingly.
Apparently if one of your staff a mod I assume sees when viewing a categories he may moderate assigned albums when viewing the category that is normal if there is a personal album assigned to that category.
You as the admin can edit that album and unassign it.
That would be under manage albums in admin.
If you do not want your users in general creating personal albums or having the ability to assign personal albums to categories then under edit usergroups in admin remove the permissions you want there for any usergroup.