I'm not a lawyer, but placing a notice up on his site is uber-wrong. You are now directly impacting his business and I've heard these people can sue you for lost revenues (or something like that).
I think the key here is communication. Before you start the project, you really need to outline what the cost is to the client and what work you'll be doing for that price. Then have them agree to the specs before you start. Then there shouldn't be too much fuss at the end of the project.
But life happens and this guy hasn't paid up. Now what? Instead of taking retaliation action of putting a notice on his web site, keep the communication channels open. Why doesn't he want to pay you the $500? Is he just broke? He get ticked off at you? Instead of $500, is there another number he is willing to pay? Sometimes, writing work off isn't too bad (you can claim it on taxes!) and you still get some money - maybe not all $500, but maybe you could get $300 or $250 - its better than nothing.
If you're really hard pressed for the $500, nothing gets their attention like a notice to show up to small claims court. If you want to mess with a lawyer, you can (and might be recommended if you haven't played with small claims). Luckily, my business partner deals with small claims pretty often so we skipped the threatening lawyer letter (and the associated fees) and went directly to small claims. The great part was the letter gets sent to the company owner. The business was headquartered in Florida and they would have had to come up to Ohio. It was cheaper for them to pay up than it was to come up to fight it in court. But to do this, you HAVE to have paperwork showing you were to do work, they signed off and approved the work, and the work was completed. Notice: "completed". If you have a note up on the site or you've taken the site down, this will throw a wrench in things such as the "work completed" (the client didn't ask for that note to be posted).
The key: communication
For next time: get it all in writing
Pay up front: be creative
For paying up front, I like to do 1/3rd up front. Our clients seem to appreciate that too.
Another story about this: Big (for us) e-comm site a client wanted. We came up with a quote. They signed it. We didn't start any work. We hadn't gotten 1/3rd up front. They send us money (about $600), but we still didn't do any work. The money they sent didn't cover the 1/3rd. About 5 months later they call checking in on us (we called them a few times but they hadn't returned any phone calls). They asked, "what's the status of the site?" We said, "Not much, you haven't paid the 1/3rd yet."
Notice here there was failure to pay. Since it had been 5 months, we drafted a new quote. This time, we've set it up with milestones. Each milestone must be paid in full BEFORE we start work on that milestone.
Lesson I learned from here was: 1/3rd up front gives you an idea how willing they are to pay (and timely). I've also learned you have to be flexible and creative with the billing or you screw yourself.
As for your situation here, determine if this fight is worth fighting for. You might chalk this one up to experience and use it with taxes.