There's really not a whole lot you can do. The recommendations here are mostly what I would recommend as well. Make sure you have a copyright on all pages, watermark your photos, etc. Making it a Flash site will not deter anyone intent on stealing your content.
Regardless of the steps you take to protect your site, if it appears in my browser, I can grab it.
I would recommend getting a lawyer to draw up a boilerplate warning letter that you can modify for each time you send it out. More than likely, people that do this don't think anyone will go to the trouble of hiring a lawyer and will scare off at the first sign of trouble. For those that don't, you have to weigh the cost of the lawyer vs. people stealing your stuff.
I would also recommend you keep copies of all pages that you alter from here on out so that you have a paper trail of changes and when they occurred in case you ever ended up in court.