I don't know if the same laws apply to grass as they apply to snow, but I am not allowed to throw my snow on my neighbor's property and I imagine the same goes for grass.

If he throws grass on your X, he must throw some on your property. You could bring him to small claims court and force him to pay for the cleaning of both your X and your driveway.

You should send him a warning in a registered letter that if he continues, you will resort to legal action. That way if ever you decide to do something, he can't say in court he's been warned.

I think I've been watching too much Judge Mattis and Judge Judy.

You could also rake the clippings and go dump them on his car.