I think it's fine to issue a bunk file to the p2p networks. In fact, I'm not sure about viruses, but the RIAA and various recording studios have in fact released copies of music that have screeching in them that can actually damage sound equipment if they're played. I'm not sure exactly how it works, to be honest, but it is done.

Personally, if you're that upset about it, I'd recommend diluting the stream with bunk files. One might recommend a texture that writes "I am a thief and a coward" in bold print across the figure's anatomy when applied.

Additionally, since the Poser community is fairly tightly knit, you could release a texture set to the p2p networks that looked legitimate, but had some telling differences, which would allow you to identify images posted with the false texture set. It just depends on how seriously you want to pursue the hurt.

My personal view, however, is that it's not worth being upset about. Most people who are going to download your product wouldn't pay for it anyway, and there's not a whole lot you can do about it in any case. Getting revenge or hunting them down may feel like a good idea, but there's no sense in letting it consume you or dissuade you or worse yet, waste time that could be better spent creating new products that will make you an income.

Give them an annoying little stab at them by releasing the false texture packages via p2p. It won't send them running to buy your product (most of them will only grab it because it's free), but it might make you smile a bit to think of the ten minutes of their time you've wasted. Then get on with the business of enjoying life. Which you should be doing with your girlfriend... A good source for how to do this would be Rotica, if you're both consenting adults. ;)