Manually clearing "database "foo" is being accessed by other users"

Hi all,

I'm finding that routinely when I try to reload a database on a server
where I know there are no connections to a given DB I get the error:

$ dropdb foo && createdb foo -O bar && psql foo -f /path/to/db.out
dropdb: database removal failed: ERROR: database "foo" is being
accessed by other users

This means I need to restart the postmaster, but the server contains
many DBs, of which some may actually be in use. How can I tell postgres
that the database 'foo' should be marked as not in use / clear or drop
any open connections / etc?

Thanks all!

Madi