I recently got a Site Template approved on Themeforest. I am getting just too much traffic on my site and noticed that my demo on Themeforest is getting ripped by some softwares
No, there is nothing you can do about this. If you have a demo you need to protect, watermark it. However, watermarks can be removed, and someone may think the watermark is in the actual product.
I would use something like TinEye.com and look for unlicensed usage of your images on a monthly basis. You can go after people stealing your work.