Starting postgres with a password

I've searched around but didn't see this answered (at least in the first 100
answers!). If the postgres user has a password (in Postgres, not the system)
*and* the hba_conf file does not trust the 127.0.0.1 host but requires a
password, how can you start up Postgresql? Can you pass this during the init
script, e.g. "password=test"?

I don't trust some of the users on a server and want to make sure they can't
just login to postgres as the admin (psql -U pgsql).

Thanks!

-doug