Managing Users

I'm having a bit of a problem managing users.

I have a database owned by user joe. Joe has a bunch of tables in this
database and is allowed to create databases but not users. All the
tables are owned by joe.
If I create another user without any privledges at all, that user is
able to (on my system) create tables in any existing databases.
Including joes database.

Do I have something improperly configured, or is this just the way
Postgres works? I realize I have little details, but I'm not entirely
sure whats causing this so I don't know what details to include.

Thanks,
Chris