How to delete all databases on Postgres?

You can do "drop cluster" and "create cluster" which will automtically erase all databases. Erase all data in you $PGDATA directory and reinit the cluster using:

initdb -D /usr/local/pgsql/data

You can use:

$ pg_dropcluster 9.2 main
$ pg_createcluster 9.2 main
$ pg_ctlcluster 9.2 main start
$ pg_restore -f your_dump_file

where 9.2 = cluster version and main = cluster name