In order to speed up our board we decided to disable forum subscriptions.  After disabling the subscriptions via the ACP, I checked the phpbb_forums_watch table and it still contained several rows.  My understanding was that since forum subscriptions were no longer allowed, existing subscriptions would be removed but this doesn't appear to be the case.  Since there appears to be no other way to remove existing forum subscriptions without playing with the database directly, it seems that the rows should have been deleted from the phpbb_forums_watch table once forum subscriptions were disabled.