To limit the categories that authors can post, you can use FLEXIaccess, get here:
www.flexicontent.org/memberships.html
Then to install FLEXIaccess properly, follow instructions here:
www.flexicontent.org/documentati
... ation.html
Note: i am co-developer of FLEXIcontent, not FLEXIaccess.
So i cannot offer support or enhancements for it, only maybe suggestions on using it
FLEXIaccess
is very powerful but has a couple of bugs.
FLEXIaccess cooperates with specific components, LIKE FLEXIcontent, AEC (Account Expiration Control) with these you can do a lot ...
For other components you can extend if you are developer ...
For backend you can limit access to all menu items, same for frontend menu items ...
IMPORTANT: also
read the documentation that is available for Flexiaccess before using (top menu documentation).
NOTE: Emmanuel, author of FLEXIaccess and head developer of FLEXIcontent is currently unavailable
Regards