I've never heard of grass fleas.....

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Googling comes up with - they're animal fleas that are just "parked" on grass waiting for a host to come along. The internet is swarming with suggested remedies - spray your garden with a suitable insecticide, sprinkle with tobacco or lime, grow fleabane or lavender, flood the garden periodically to drown them....
It's not a problem we get in the UK so far as I know. I'd be interested to see what people in warmer places think.

Obviously some critters out in Korea, Beattie.

Sorry we can't help, Graham. :-(

Ortho makes a 'Ant, Flea, and Tick Killer' for lawns that you just spray on. I did a quick Goggle search, and most sites advise you keep the grass cut and regularly flood the area with water to kill the larvae. They say it is caused by fleas falling off pets and then living in your garden.

Do you have pets...Perhaps they need fleaing more regularly. Do you have a dead hedgehog somewhere in the garden (does Korea have hedgehogs?) the fleas will jump off the carcass and wait for a new host.

But take heart if they do because although fleas will bite anything with blood in it they can only breed if they bite their own host animal. (eg hedgehog fleas need to bite hedgehog before they can reproduce)

Is spinosad available in Korea? If so, it may be the safest control method. Any method you use, though, it would be a good idea to find the source of the pest.