How do we advertise and market a business conference without cold calls?
(1.) You've specifically excluded personal selling. You don't want to make cold calls.
(2.) You tried online, but didn't get much interest; just one twentieth of one percent click-through.
(3.) Word-of-mouth didn't work, either, so
(4.) You've concluded you must be using the wrong vehicles of message delivery.
Have you considered that you might just have a dog of an idea?
Advertising works best when you have something to say. Is your offer compelling? Is the event credible? What are your credentials? The poor return on your initial investment of effort indicates to me that your problem is most likely in the message itself. Craft a better event. Secure better speakers. Offer a stronger package. You'll be surprised at how well online marketing and word-of-mouth work when your message to the public is attractive, surprising and credible.
Good luck to you.