How can one prevent a co-author from publishing without consent?
Write an email to the editor.

As you said, submission needs formal aproval of all coauthors so if you tell the editor or publisher that this is not the case, the paper will not be published.

Publishers usually assume academics are grown ups and the likelihood of person naming people as co-authors against their will is very low and thus the hassle of sending multiple emails or having everyone involved in the online submission is not worth the trouble. Keep in mind that some articles have up to 1000 authors.