Ultimately there's nothing you can do to stop a truly determined mod thief from stealing credit for a mod you made, but there's nothing stopping you from making it as difficult as you possibly can.
1. Lock the .3DG file when you export from ZModeler2. This will prevent someone from using your mod as a basis for future projects. Locked files can potentially be unlocked, but it takes someone fairly skilled to do it, and it doesn't look like the current crop of thieves has that level of skill.
2. Sign the textures you use on a section not used by the mod. Every texture has unused space, use it to identify yourself as the original modder. Unless the thief is looking for the signature, he'll leave it in there which guarantees you a way to identify the original modder. This is especially important for HUD creators, mark your textures in a way that can't be seen ingame, but CAN be found if you investigate properly, because HUDs are being stolen as well as car and bike mods.
3. Watermark your screenshots. Find someone with photoshop to make a transparent text overlay of your name across all your preview images, right over the car/bike itself. Stop the thief from using your own screenshots for his gain. In a pinch you can use Irfanview (<--Clicky) or even MS Paint to leave opaque text across the image.
4. If you use an auto-installer, leave your name in it somewhere in the install.pch file. Most of the thieves who leave the installers intact never check that file, and it's just one more way to identify that it's yours. For Sound modders, if you can hide your name somewhere in the CarVSTConfig.xmb file, do it, or if it's possible to leave a readme TXT file inside the sound BNK without corrupting it, do that, car sounds are being stolen along with all the rest.
5. Leave your own name or logo on the outside of the car mod in 3D letters. I know it's not "realistic" to do this to every car, and I know some of you don't like this when sites like HF Garage does it, but if you lock the 3DG file afterwards, it's a highly visible reminder of who the original modder is that can't be removed. Just think of it as a "Dealership Logo" that most car dealers stick on cars they sell every day. You could even leave the text/logo in body color to hide it in plain site against the car body, it would be visible from an extreme angle in a reflection, but nearly invisible when looking directly at it. You could even make it small enough to be passed over on initial inspection or hide it in a specific location that makes it nearly invisible.