Is it possible for me to hide the entire category drop down list while writting or editing an item except for the ones an author from a k2group has the right to access?

havent tride it with 2.6.1 but something similar to these approaches should work
williamwhitejr.com/index.php/k2-tips/item/92-how-to-disable-unused-categories-in-front-end-editing

:) believe its gonna help me out already feeling happy. I will tell you more