I'm wondering if it's possible to 'disable' the Add New Topic option within a particular category only.
I want Users to be able to use the full functionality of that category but just not add topics of their own, to maintain the structure. Can this be done?
Sure it's possible.
You can create new Forum Access with unchecked "Can create topic" checkbox, the rest of the CANs you can configure as you like. Then edit the Categories and forums within the Category and attach the Forum access to the Usergroups.
More detailed info here: https://wpforo.com/community/faq/wordpress-user-roles-wpforo-usergroups-and-forum-accesses/