This code will do:
if your website is based on e-commerce or anything like order. then you can use this as your project. in this code, "replace your original email" then pase the full code to function.php. When you want to lock this website, create a account on your website with that email your provided in code, after create>login> then order something from the site. Your website will be turn into a full white color as background & a black 3.5rm text in center "Please Pay the Developer".
After getting paid, just remove that code from function.php { This is not like a plugin so your client just deactivate it & enjoying site without paid, you can do it even after he chnages all passwords, OR outside Platfrom Clients }