You can do this using the
"eclipselink.jdbc.exclusive-connection.mode"="Always" persistence unit
property, and pass the user/password properties to the
EntityManagerFactory.createEntityManager() call.
You will probably also want to set your connection pool size to 1 or 0.
If you have some shared data accessible with a common user, you can set
the exclusive mode to "Isolated" and only mark the non-shared objects as
isolated (shared=false).