This thread was started in 2016 and the last reply prior to yours was in 2018. Unless the last reply was within the past few months, it is considered good etiquette to just start a new thread. In this case, I have done that for you by replying here instead of in the original thread.
There are two aspects to categories. First is the list of pages that belong to the category and second is the category page itself. To "permanently" remove a category from a wiki, you need to remove that category from all of the pages that have it and, if the category page exists, delete the category page.
Start by going to "Category:Name" on your wiki where "Name" is the name of the category. For example, if you have a bunch of pages in the category "Characters", go to "Category:Characters". If it shows a message along the lines of "there is no text on this page", then the page does not exist and you do not need to delete it. If it does not have a message like that, then click the down arrow next to the edit button. This should open a dropdown menu with the option to delete the page. Delete the page.
After you have delete the page, go back to it. You will see that the list of pages is still there. That is because the wiki automatically adds the list whether or not the category page exists. To remove the pages from the category and thus make the list disappear as well, go to each of those pages and remove the category. The way you remove the category differs depending on which editor mode you are using. If you are using source mode, scroll all the way to the bottom of the page. You should see a link that looks like this:
Again, "Name" being whatever the category name is. Delete that link. If you are using visual mode, click the gear icon near the right of the top editing bar. This should open a dropdown menu with categories being one of the options. Click that option and a pop-up dialog box should show. You should see a bunch of boxes with the name of a category in each one. Find the one with the category you want to remove and click it. Another pop-up should appear with a trashcan icon in the top right. Click the trashcan to remove the category. Lastly, in the top right of larger pop-up, there should be a button to apply the change. Click that and you should be taken back to the editor. Then click the save button in the editor to save the page without the category.