I've read this type of scenario on many forums. Poor designers getting fucked by tightwads.
I know this one chick who redesigned a site for two old ladies. Their old site was loosing 20k a week because something was wrong with the cart system.... but you know... their old and don't know these things.
Long story short, they used one of the old ladies computers to view the work they did, they didn't pay, and one of their grandsons scraped the cache files and (poorly) pieced it back together.
Anyways, if there are any people here that know how to write software, then you got your next big winner -
Write some software that will allow a designer to log into a host, and stream the necessary information to any computer they happen to be in front of. This way they can show the client the work they did, in any location, and NO files will be cached.
For website design, you'll need something that can stream the entire site, in it's full glory, AND it must somehow be functional.
A system like this would allow all designers to keep everything on THEIR servers. If the client is really 'wowed' by it, then they just gotta pay the bill before everything is uploaded to their host.
During this whole thing, designers need to keep in mind only one thing... DON'T tell them how you created their 'product'. Just smile and say "Meh... I've got a few tools"
Certainly, you're going to lose some customers, but at least you won't have your work blatantly stolen from you, and they won't be able to tell their 8 year old how you did it.