database security

How do I ensure that a user cannot open someone else's database and
create tables there?  Right now all users have to use passwords and
they cannot see each others tables when they log into someone else's
database but they can create tables which the owner of the database
cannot remove.  This is annoying to me.  I don't want to have to warn
people that they will lose their priviliges.  I would like postgres to
not allow this.  Any advice?

gabor.
--
    pos += screamnext[pos]  /* does this goof up anywhere? */
        -- Larry Wall in util.c from the perl source code