Para o PostgreSQL 9.1 ou inferior:

SELECT pg_terminate_backend(pg_stat_activity.procpid)
FROM pg_stat_activity
WHERE pg_stat_activity.datname = 'NOME_DO_BANCO'
  AND procpid <> pg_backend_pid();

PostgreSQL 9.2 ou superior:

SELECT pg_terminate_backend(pg_stat_activity.pid)
FROM pg_stat_activity
WHERE pg_stat_activity.datname = 'NOME_DO_BANCO'
  AND pid <> pg_backend_pid();