J.P.
Consistency is key... keep up with the grounding and taking his possessions away. Sooner or later he will understand that his actions come with consequences. When babysitting my nephew, if I took a toy from him due to his bad behavior, he had to earn it back with good behavior. For him, it worked a lot better than holding his items for a certain amount of time. Don't be afraid to turn around and take something back either. If he gets mad and gives you an attitude, give him the option to explain his actions in a respectful manner. Maybe that will 1. show him that his actions were not justified 2. shed some light on what might be really bothering him and 3. teach him to handle issues with respect, and patience. It probably will take a while to get the picture clear, act this way and lose your privelages, we can either talk about it and you can earn it back or you can learn to live without. Don't let up. Also, the best advise anyone has ever given me is this: the most powerful word in the world. You have every right in the world to say no. Not just to your kids but to anyone. Saying no gives you power over yourself.
I hope this helps you as much as it has helped me... good luck!