Permission and users

Hi,

PostgreSQL superuser (named A) create two different users:
B: able to create db
C: able to create db

B creates db B_DB
C creates db C_DB

C is able to add tables at C_DB as well as at B_DB.
B is able to add tables at B_DB as well ad at C_DB.

How can I limit C to operate only to C_DB
and
B only to operate on B_DB?


I can modify permission on C_DB to avoid read,modify for B user, but I'm not know how
to exclude B to access to C_DB at all.


Thank you in advance for any reply.          \fer