Also you can setup a couple of Google alerts with things like your home page title, a unique piece of text from you page in quotes etc. It wont "prevent" the problem but at least you will be emailed the moment one of these proxified pages gets cached so you can take action before much damage is done.
great idea. Maybe even implant a bit of text code that means nothing and has no current pages when searched into the page. Them, do as sweetfunny suggests on that text code. Phoneguy, do you mind PM me the site url? i want to compare the symptoms to mine. Also, i want to ask you something via PM.
Yep i do that quite a lot, for example a SEO experiment i just finished it was important nobody copied the page so i had the word "Yailodo" on the page which had no results and a Google alert on it. It's very effective not just for proxy hijacks, but sites in general copying your pages.. Also when you make new pages, you get notified when Google indexes it which can help knowing trends on your site with how long between page creation and indexing.
Dont be so quick to jump to the spam idea. There are accidents that happen as one happened to a proxy of mine mid last year. For some reason I left out a no follow and allowed googlebot to crawl everything. The mess was crazy. googlebot decided to index the web through my proxy. I had no idea it was happening since the site was on autopilot so to speak while I went ahead with my full content site and was getting ready to switch hosts. It caused a mess and once I was alerted to it I took my site out of the rankings asap to fix the problem. It happened to one of three proxies linked together so Im not quite sure why. I got the typical accusations of scraping and was reported to my host at least. However if you search about the subject(its been over 6mo from the incident so im not quite sure to the term) its decently documented what happens. If you allow a spider to crawl the whole proxy and not just the "start" page it will sometimes get stuck and index everything through that proxy. This is what happens. I'd start with an email to the webmaster and host. It could be an accident... might not be but it could.