Help with privilege or pg_hba.conf

Hello!
I have a Unix box running PostgreSQL 7.3.3.

So far, I have been the only one user of the DB server...and I didn't have
any problem using it with the JDBC driver.

Now, a friend of mine wants to use the DB too for doing web stuffs(PHP,
JSP).... so I need to create a new user on PostgreSQL. this has been done
easily.
Now, I don't want the new user (his username is "cool",and he has a
password ) to access other DBs on my server... I want him to access ONLY a
database named "cool" that he owns
(as user "cool", I have created the DB "cool"... and I want  user "cool" to
access ONLY  DB "cool" and do any operation on it).

How can this be done?

I've been trying to modify "pg_hba.conf" but with no luck .... as user
"cool", after I connect to the DB server via psql and I do "\c anotherdb",
user "cool" can still connect to "anotherdb".
my "pg_hba.conf" can be found at http://ahouans.sh.cvut.cz/pg_hba.txt

Thanks in advance.

Have a nice weekend!

Arcadius Ahouansou.