If a horse is pushy with his head and is intruding into your space, pushing his head away will only send him a “game on” message.  Simply put your hand/arm up so that it aligns with the corners of the horse’s mouth (where the bit  would go) to create a block or boundary.  Do not push the horse’s head.  The horse may try and push at your hand/arm to come into your space, however stand your ground with your block.  Pushing back at the horse’s head is a “bully” approach.  Establishing a simple, non threatening block will create respect and build trust.