Be careful with this. If you intentionally implement a back door in a work product you're developing for a client, and the client finds out about it, you may find that the trust between you and your client will be destroyed. Also, there may be legal repercussions if disabling the product or application has a negative impact on the client's business.
Also, consider that there must exist some degree of trust between you and the client. They're trusting you to deliver, and you're trusting them to compensate you for that work. If they discover you've intentionally written a security hole into their application, you may find yourself out of work, branded as a fraud, and unable to convince others to trust you.
But lets say they do delay or withhold their payment. Remember, there's more at stake than just your money, there's also the livelihoods of the employees who work for the client, and the businesses and livelihoods of the clients who rely on your client to provide services. If you do something to harm your client's business, you may unintentionally do more harm than you anticipated. I can't say for sure if there would be legal consequences or if third parties could file a case against you, as I'm not trained in law, but I would personally have trouble sleeping at night if I did something that put some innocent person out of a job.
Consider that this is just a cost of doing business. Some clients will pay, and some clients will not pay for a wide variety of reasons. Some may pay months later after you've sent monthly, professional past-due notices as a reminder. The best way to protect yourself is to make sure that you bring in more revenue than your expenses so that you build up a cushion of cash so that one deal gone bad won't put you out of business.
In fact, this happens to businesses all of the time: They become tight on cash due to cash flow problems and can't pay their debts. But once they dig themselves out of their predicament, they will likely work on paying vendors who they owe outstanding balances. These established businesses don't get mad or launch attacks against these parties; instead, they remain professional and continue to approach these collections legally and professionally.
If you're going to work as a contractor, you'll be working in a professional environment. That means you must conduct yourself with the same level of professionalism that you expect from your clients.