Separating data and users is a key part of administration. There will always be a need to give users a private, secure, or simply risk-free area ("sandpit") to use the database. Here's how.
Again, make sure you've read the Deciding on a design for multitenancy recipe so that you're certain this is the route you wish to take. Other options exist, and they may be preferable in some cases.
postgres=# create user fred; CREATE ROLE postgres=# create database fred owner = fred; CREATE DATABASE