I've heard some can be adjusted shorter by removing the bumper and turning down the shaft of the shock. I didn't have any luck doing it that way.
With the bumper still on, just put some rubber gloves on and drill a small hole in the body of each shock. It will Pssst! oil out all over the place.
Take a piece of 2x4 place it over your bumper, then hammer the wood to knock it in.
To get the bumper off you'll need to yank the bumper back out, as tucking covers access to the bolts.
Performance one step at a time.