Is it ethical to submit the final version of a paper after acceptance when coauthor refuses to agree to submission?
I checked the authorship guidelines of the two most important computer science societies, IEEE and ACM. IEEE's guidelines have a clear answer to the question, although I wish it was a different one, given the unfair situation the post-doc has put you into:


  "Each individual named  as an author must approve the final version of the article as  accepted for publication, including the references." (Source: Sect. 6, "Author Responsibilities")


In the case of ACM, I didn't find a direct answer. The closest statement I found was the following:


  "Once a submission has been accepted, [...] ACM expects authors to speak with one voice even if there are multiple authors." (Source: "Processing of accepted works")


(Not a particularly helpful statement anyways: there are certainly situations where an individual author would behave ethically by not speaking for all authors, like whistleblowing).

In any case, you should look up the authorship guidelines of the journal where your paper was accepted - they might have yet another relevant policy for your case.