The guiding principle should be to never break anything the customer already has. That is likely to be illegal in numerous jurisdictions. Moreover, never do anything unfriendly to your customers. That's basic PR. It also can be a matter of saying things in the right way. An extra charge on top of your standard rates is a lot more annoying than higher standard rates and a discount for some customers.
Given enough work, the chances of getting stiffed approach 100%, and you do need to account for that.
If possible, build the chance of getting stiffed into your rates. For repeat customers who have a history of paying on time and in full, give them a preferred customer rate, which would be your standard rate minus the amount you were reserving against the likelihood of not being paid.
Make sure you have a written contract that covers payments and deliverables. Make sure that the payment schedule doesn't allow arrears to build up past what you're willing to accept losing. Make sure the contract has some sort of linkage, so you aren't required to keep working without payment. You should draw up a sample contract and run it by a lawyer. A lawyer up front will cost a lot less than a contract dispute, and contract disputes tend to come up when the contracts are unclear. You never want to be in a position where a court has to figure out what the contract means.
Never talk about disabling things in any manner. That will get you in trouble no matter how bad the customer. If you're worried about it, put some sort of license control on the demo and trial versions of the product, and have it in the contract that you will turn over the permanent key at the time of the final payment.
Given the tone of the posting, it appears that the OP may already have been stung badly. If somebody has, accept it as a learning experience and make sure it doesn't happen again.