Restrict access to system tables

Please pardon the novice question, but I can't find my answer in my
books or the docs.

I want to keep as much data and table structure hidden from users as
possible. I have created a function to return data (using 'security
definer') and revoked all rights to a table for a user. This user can
now execute the function but not query the table directly... Great.

BUT this same user can use PgAdmin III or psql to view the "schema" --
everything except seeing the data.

Anyone have a suggestion as to how to limit a given user (a group
preferably) to only executing functions created by an administrator????

Thanks