Depending on the circumstances, you have a few options.
If you do not want the post to be published at any time in the future, you can simply delete the post from the Content Calendar.
If you think that you might want to publish the post in the future, perhaps after making changes, you can move the post to draft format. Be aware that if you move the post to a draft format, someone else in your organization with publish permissions might publish the post. As best practice, be sure to include a note explaining why you canceled publishing for the post.