CSS Killswitch
http://csskillswitch.com/

CSS (short for "Cascading Style Sheets") is the markup that makes the internet look the way it does. CSS Killswitch works by creating a unique, inconspicuous, and innocuous stylesheet that you'll link the potentially problematic client's site to. Should negotiations go south and passwords get changed before you receive payment, that stylesheet can be activated to superficially black out the site, returning to you the upper hand. * When relations return and you've received payment, the killswitch is easily deactivated and its traces removed. Not convinced? Try a two second demo of what it could do to this page. Oh, did I mention this is free? Because it's free.

CSS Killswitch lets you non- destructively black out a difficult client's website with the click of a button.

Web Developers: Have you ever gotten to the end of a project, and had a client withhold the last of your fee to exact additional changes or features that were not in the original plan? Perhaps a client that decided your work “wasn’t what we expected” and tried to withhold payment? Well worry no more. Put the power back in your hands with a Remote Kill Switch. The idea is this: you build into their website a small function that checks with a server you control to make sure the client’s account is in good standing. If it is, the site loads as normal. If not, their site doesn’t load, and they get a message asking for payment.