If "respond" means to give an answer:
* Ignore: To pay no attention to something or someone.
* Disregard: To ignore something or someone.
* Neglect: To fail to care for or attend to something or someone.
If "respond" means to react to something:
* Ignore: To pay no attention to something or someone.
* Remain indifferent: To stay unconcerned or unmoved by something.
* Be unresponsive: To not react or show a response.
If "respond" means to act in a certain way:
* Refrain: To avoid doing something.
* Decline: To refuse something.
* Resist: To oppose something.
To choose the best antonym, consider the specific context of the word "respond" in your sentence.