Questions on PostGreSQL Authentication mechanism...

Hi, we have latest PostGreSQL setup and it allows everyone to connect. When
I do \du, it gives following output and it is same for all users.

TechDB=# \du
List of roles
Role name | Superuser | Create role | Create DB | Connections | Member
of
---------------+-----------+-------------+-----------+-------------+-------------
sonal | no | no | no | no limit |
{from_ldap}
sundar | no | no | no | no limit |
{from_ldap}
...
..

Moreover, anyone can connect to databases as postgres user without giving
password.

I am not aware how above setup has been made but I want to get rid of them.
Could anyone please help me in below questions?

1. When user connects to TechDB database(or any other) as a "postgres"
user, it should ask for password.
2. Remove all above users(listed with \du) and create someof users and
they will have only table creating privileges.

Thanks.