Aggressive Play

My pup is a little over 7 months old now and loves the dog park. The problem is that he has started to play very rough. He will grab on to a playmate by the neck or ear and not let go. The more submissive the other dog, the rougher he gets.

This started about a month ago. Prior to that there was always a lot of chasing and doggie wrestling, but no "death grips".

We breakup this behavior immediately, but he always seems to return to it.

How do I get him to "play nice".