General antonyms:
* Omit: To leave out or not mention.
* Refrain: To hold back from doing or saying something.
* Suppress: To prevent from being expressed or released.
* Withhold: To keep back or refuse to give.
More specific antonyms:
* Agree: To express concurrence or approval (if interjecting a disagreement).
* Support: To express agreement or encouragement (if interjecting opposition).
* Ignore: To pay no attention to (if interjecting a comment).
* Allow: To permit or let something happen (if interjecting a prohibition).
The best antonym will depend on the specific situation in which you are using the word "interject."