how to prevent some user group to post in a category

Hi. I want that only users belonging to specific usergroup can post in a category. So i try to set category - permissions and choose privacy field like Assigned Joomla Usergroup but when i edit it and click on save i receive the message i cannot save it because this is default category and not public. Why?
Also how can i set only 1 language as predefinied and none can change it?
Thanks, best regards