The "nicknames" are linked to several traits. As Eothese wrote, these are given by the epithet lines. If you want to modify or remove these, then the files you're looking for are:
\data: export_descr_character_traits.txt
Open it, and with a simple ctrl+f, search for epithet, and you will see which traits can give nicknames for your characters. Here you can change any conditions, benefits, etc.
Trait looks like this:
Text file:
\text: export_vnvs: as in the above post, you can change the texts themselves to anything you want.
As in the above example, this is what you are looking for here: