I have watched the video on Oracle General Ledger > Managing Categories (how to create new Categories). Could someone tell me how to remove some categories from the list of all categories showing up when entering journals? Is this possible, and how to achieve it?

You cannot do so. Enhancement requests have already been logged with Oracle for this issue

This is what Oracle says about the issue:

There is a workaround suggested to address the above problem:

Change or prefix the obsolete seeded or user-defined categories with the word 'Obsolete' or 'Do not use'
e.g. Change old source name "Feeder" to new source name "Obsolete - Feeder" or "DO NOT USE - Feeder'

Alternatively, if you want the obsolete journal sources or categories show up at the bottom of the LOV, user can prefix starting with Z e.g. Z-DONOTUSE Feeder.

It is to be noted that this workaround should be implemented after ensuring that the there are no entries in the concerned sources or categories.

Thanks Hasan. I am very grateful!