I usually refer to the whole copyright claim as "the nuclear option": once you use it, there's no going back. Make certain that you know how serious your claim is when you make it, and follow through on it. While it does serious damage, in my opinion, it also strikes a fear of god into the minds of clients who are often far too cavalier with their dealings with cameramen.
I've had to use "the nuclear option" three times now. Here's a copy of a portion of the email I sent the production company that failed to pay me per the terms of my deal memo. I was told this was forwarded to the individuals up the stream who were dragging their feet or withholding payment from the production company with whom I was working, and payment was remitted within 24 hours:
I am given the impression that the agency or [COMPANY WITHHELD] (or both) have failed to fulfill their financial obligation to you, which results in failure to pay [my 1st AC] or myself. I don't feel comfortable continuing to delay the terms of our agreement for the work performed over a month ago. I honestly do hope that [COMPANY WITHHELD] or [COMPANY WITHHELD] was legitimately delayed for simple reasons, and by next week this will all be water under the bridge.
However, I feel obligated at this point to make clear my intentions.
If by Friday, January 17, 2014, I don't have some material assurance that my payment is en route, I'll have no choice but to draft a cease and desist letter to [COMPANY WITHHELD] as well as [COMPANY WITHHELD] for failure to pay resulting in copyright infringement, namely all footage I shot used on [COMPANY WITHHELD]'s current marketing campaign.
My cease and desist order will be accompanied by takedown notices at YouTube for the relevant videos currently on the [COMPANY WITHHELD] channel, which collectively have close to 200,000 views.
Please don't misinterpret this as anything personal. The last thing I want to do is sour a blossoming relationship with two people I enjoy working with professionally and personally. I don't like the situation we're in, and I tried to avoid it...
I hope this serves as ammo with which you tell [COMPANY WITHHELD] that payment needs to be made, OR [COMPANY WITHHELD] needs to make other arrangements to fulfill the obligation to [my 1st AC] and I so you can continue to pursue [COMPANY WITHHELD] and [COMPANY WITHHELD] as necessary how you see fit. Only satisfactory completion of the terms of our employment will avert this action, requiring some material assurance of payment to be made by Friday, January 17, 2014.
The moral of the story: having a lawyer draft a cease and desist letter is expensive. Threatening to have someone ready to write a cease and desist order and send it to whatever purchasing agent they may be discussing distribution possibilities with works great too! When negotiating the distribution, the property carrying debt and potential copyright liabilities can drive down the purchase price.
Aaaaanyway, please don't use my email as a template. That was a very friendly version of what I've threatened elsewhere to companies which I had no allegiance, personal or professional. The importance is conveying 1) you're right, and 2) you're serious. Until you're satisfied, they don't have property to sell, and that's a very unstable position.
The cost of a copyright action is EXPENSIVE, and it's a slam dunk for you. They have every reason to just pay you what you're owed, which is definitely cheaper than the lawsuit.
In the words of Joe Pesci: "F**k you, pay me."