Disable/Enable global categories like ideas

Ok. You can temporarily remove those categories:

1. Go to the category, by going to the homepage, then click Categories then Ideas.
2. Click Edit Category to the right.
3. Click Unlisted and set it to unlisted. Hereafter, only staff will see it.

Or you can delete it (click Delete in the edit category dialog), and later on, when you need it again, you can undelete it. If you delete it, however, you won't see it any longer yourself (unless you choose "Show deleted" in the topic type dropdown). Maybe then eventually you'll forget that you can undelete it. So, unlisted, is maybe better in this case.

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In case you also want to hide the topic type buttons (buttons that let people choose among Question / Idea / Problem / Discussion) — then, go to the admin area: Click your username menu, then Admin. In the admin area, click Look and feel, then in the Basic section, uncheck these settings:

 Show topic type icons [x] Uncheck to hide topic type icons in the forum topic list
     Choose topic type [x] Uncheck to hide choose-and-change topic type buttons

However, you and other staff members will still see these buttons. Only "normal" members won't see them. To see how the forum looks, for a non-staff member, you can click your name menu, and then View this site as... and view it as a not logged in user. Or, you can go to a user's page in the staff area, and click Impersonate, and see how things look, for him/her.

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Maybe you want to remove categories completely? Then, in the Look and feel section (see above), uncheck Use categories.

It'll be interesting to hear how this works for you? & please feel free to tell me / ask more questions, if something is unclear, or doesn't work.