Italicised keywords are keywords which exist in the metadata of selected images but are not part of your keywords list.
Various ways exist of removing them but do keep a backup first.
If you do not want these keywords added to your keywords list, ensure that no images are selected then simply click on Other Keywords and delete. However this does not delete the keywords from the images to which they were applied and as soon as you select one of those images, the Other Keywords category will reappear. To rid yourself of a keyword altogether you would have to search for all images containing that keyword (Find or Ctrl+F), select them all and uncheck the box next to that keyword so that it is no longer applied to the images. Then when you delete it, it will not reappear.
If you do want to keep these keyword, right click on them, one at a time, and choose “Make Persistent”. Then you may drag each keyword into an appropriate position within your keywords list.
Hope this helps.