* Ignore: This is a general antonym, meaning to pay no attention to something.
* Disregard: Similar to ignore, but with a stronger implication of deliberately choosing not to listen.
* Dismiss: To reject or consider something unimportant.
* Disbelieve: To refuse to believe or accept something.
* Dissent: To disagree or express opposition.
* Defy: To openly resist or disobey.
The best antonym will depend on the specific situation and the meaning of "hearken" in that context.