Role based database access

Hello!

I face a problem here with restricting access to databases to
the owners of the database (Postgres 8.1.4).

We would like any postgres user to have database access restricted
to their own databases only. This is so, as we use postgres for
educational purposes.

I looked into pg_hba.conf, but unfortunately, when restricting
database access in the database field, the NAME(!) of the database
is concerned, which we do not restrict. Any user can create as
many databases as needed, and the name may be choosen freely.

The 'user' field does not help in this either.

So is there any chance to achieve what we need without revoking
the 'create database' permission and pre-creating the databases
for all our users?

Thanks,
Thomas Mack