Hello
-- first keep in mind that
Joomla ACL has heritage !!
1. so DO NOT alter the category ACL permission (
if you have done ANY changes to categories then set these back to "inherit")
2. Only set the ACL permission in FLEXIcontent --component-- configuration (it is the "Configuration" button at top right)
find your desirable user group e.g. "registered" and SET:
1. deny for: edit, publish, delete
2. allow for: "create", edit own, publish own, delete own
... these will be inherited by ALL categories !!
3. FINALLY if you have some categories that you do not want to allow user to create , place these under a single top-level category and
FOR this top level category set DENY for "create" privilege, this will prevent user to create (=assign) items to this top level category and to all it's subcategories ,
Here is Joomla's official tutorial
docs.joomla.org/J2.5:Access_Cont
... t_Tutorial
Regards