varying the height of the jumps in the grid or increasing it towards the end and gradually reducing the distance makes them pay attention more. but do it gradually! rushing through grids is usually due to lack of experience, confidence or strength
I agree with the ground poles and cavalettis. Not the stopping before the jump, that can get dangerous. Use grids/ground poles that change direction (ie figure 8, four leaf clover, etc) and asking for different bends, and switch up the directions. Keep the horse thinking, bending and staying aware of its feet. And stay out of its way! Let it do its job. My gelding started to slow down with these different exercises and when I just let his mouth go and let him do his job over the fences. Hope you get the help you're looking for!
Try stopping in front of the jumps. Canter towards the jumps as if you would jump it and then halt like, 1-2 metres in front of it. Do it over and over, until your horse gets it. Halting in front of the jump, when your horse rushes it is one of Pro rider Michel Roberts' tricks. It works well with my horses. Also make sure your knees don't cluch the saddle, causing pressure on your horse. That might send him forward. Sit back and keep your hands low, no matter what. And be aware of your breathing. Slow steady breaths. Thats really important! :)
I know something people use lots of ground poles before, during and after the grid to make their horse think about where it's putting its feet, and also put ground poles or cavaletti in a circle to make the think about bendkng and working at the same time. My horse doesn't rush through grids so I have no personal experience with it, but many of my friends have had that problem. :)