One dangerous thing that stands out is this "kill switch". VERY bad idea. Now you are liable for damages.
The way to handle this is: When you get the judgement against your "client", slap a lean on his properties equal to the amount owed (and throw in your court costs). That inclues his accounts. All you need is an order from the judge. That's it.
That's about all you can do really. Is to go after his assets. Also I'd let the BBB and your local Chamber know. You'd be surprised how effective both these organizations can be.
I wonder if it's legal to state upfront in your contract that 'technical measures' are implemented in your work to prevent theft of service (or whatever you call the use of the work product without payment), and are to be deactivated & removed following completion/final payment?
The software licence-key approach seems somewhat similar, and I've yet to come across legal proceedings "because we didn't pay the guy and our 30-day demo ran out", although I'm sure it could get more complicated for services rather than actual shrinkwrapped products.
On the other hand, you've got the fact that it's laid out in the contract (which is presumed read, rather than just clicked through like a shrinkwrap EULA), and the interesting potential for DMCA/WIPO96'alike anti-circumventation proceedings against clients who refuse to pay and extract the kill-switch code to reactivate.
In that theme, is:
final_invoice_paid = false;
if (!final_invoice_paid) { exit() }
a valid "means to descramble a scrambled work, to decrypt an encrypted work, or otherwise to avoid, bypass, remove, deactivate, or impair a technological measure, without the authority of the copyright owner"?
My immediate thought is yes, assuming you're still the copyright holder (and you have a copyright assignment on payment clause in contract), but jurisdictional work-for-hire terms might screw you.
Reminds me of the talk "Fuck you, pay me", where they talk about how the client only gets copyright assignment upon reciept of final payment. Thus, if they stop/don't pay, then they do not have copyright on the work, and hence are guilty of copyright infringement if they use it.
It is always a good idea to keep ownership until final payment. This also increases the chance that you'll see money when a client goes bankrupt and is sold or undergoes a restructuring ("doorstart").
You can structure the "kill switch" as temporary licenses that expire after a while. Give them an 1-month license to try out things. Give them the full license upon receiving of full payment. If they want extension, give them another 1-month license.
The way to handle this is: When you get the judgement against your "client", slap a lean on his properties equal to the amount owed (and throw in your court costs). That inclues his accounts. All you need is an order from the judge. That's it.
That's about all you can do really. Is to go after his assets. Also I'd let the BBB and your local Chamber know. You'd be surprised how effective both these organizations can be.