This post makes your company look considerably less professional than it otherwise would have, even leaving allegations about code theft unanswered.
I might have been unsure if you were running copyrighted code, but now I know for sure you stiffed a programmer and are trying to cover your ass after the fact.
As a professional consideration, I won't be using any of your services. Failing to pay an appropriate invoice for services rendered to you is a serious black mark for a company, particularly to people who depend on contract work to make a living.
Finally:
> Our agreement with the poster makes it clear that we own all work product produced pursuant to the agreement
I suspect that this is only true in the event that you completely paid the programmer. Failing to do so likely invalidated the copyright transfer, which is standard language to include in a contract.
I also suspect that the code you're currently running is not a clean rewrite, meaning that the next programmer based his code on that code and likely didn't remove literally every piece of it from the code base before starting.
This very easily could have left your company with liability regarding the code you're no longer using, because the formation of your current code base depended integrally on violating the copyright of the programmer you didn't pay.
I suspect you should just shut up and stop making a bigger deal of this in public, and that you should ask your lawyer point blank if the cost of fighting over the liability you might not have properly controlled will be cheaper than just paying the rest of the programmer's fee.