You can divide them or root-prune them. The techniques and timing vary somewhat, based upon the kind of grass each one it, so the first thing to do is ID them. Then closely examine the crown of each one. If larger warm-season grasses are starting to 'donut' (die off in the center) you'll make divisions using only the outer perimeter of the crown.

Cutting them back is seldom desired, especially if they are large warm-season grasses. It leaves the ends of the foliage squared off and usually browned, and just doesn't look right.

Ultimately, the better solution would probably just be to replace with more compact varieties. Even in the larger grass species like Miscanthus there are shorter and taller varieties.

Yes. If they're 8 feet tall, they will always be 8 feet tall, no matter how much you divide the clump.

I've started using a dividing technique that makes the process less daunting. I think of the clump as a big pie and cut it up into pie pieces using a recipracting saw, then dig out each 'pie piece' by first digging from the outside of the 'pie'. So quick. So easy. I use an old blade I don't really care much about just in case I run into any stones. Try it!

Replace them with shorter versions of similar grasses.

Dividing them won't change their height.

If you take a division from the perimeter of an 8' Miscanthus, it will take 2-3 years for the plant to reach mature height again. It's like starting over with a young plant.

True, but doesn't reduce mature height.

Make the hole deeper, give them a sunken living room.