Ways to block programs on freelance?

Question

Freelance

What are the ways to stop the program at the end of time?
I know that it is necessary to draw up a contract, work through save sites and so on (not freelance at all), but it is much more effective to casually mention that if the client does not pay the amount, then everything will crash in 2-3 days.

It occurred to me only to create some kind of variable and, when the application starts, initialize it depending on the time. Well, block the launch or some functions, depending on its value.

What methods do you know to do this?

Answer 1

Move the critical part of the business logic to an external server. This is better than authorization on the server, because if you can intercept the request and change the server response, then you cannot replace the work of a critically important method. After payment, you can download a plugin that does the same, only locally.

Answer 2

Staged prepayment

Answer 3

Take money strictly in advance, work under the contract, personally communicate with the representative of the customer and his management, draw up and certify the exact technical specifications.
And you don't have to yell.

Answer 4

To say in advance that this is a demo version for a month. Then a password request. You send the password upon full payment (you can do it ahead of schedule).