I've also seen it suggested to put some text like "This description was stolen from [sellername]" in the body that's the same color as the background. The thinking is that the thief will copy and paste and won't notice when it defaults to black after they paste (assuming it does).
You're reporting these incidents to eBay, right? In my experience, they'll kill listings if you can reference the one they're copying and show that it was listed first.