Fleas go through four life stages: eggs, larva, pupa and adult.
A lot of sprays kill adult fleas but don't have much (or any) effect on their early stages.
The conventional two-pronged attack on flea infestations usually involves an insecticde followed up with an insect growth regulator (to stop larvae turning into adults).
Most of these chemicals are toxic to humans, their pets as well as fleas (and many innocent helpful insects plus spiders).
I have heard that spreading common salt can be helpful indoors (it dehydrates the buggers). Not sure how it would work outdoors, especially during the wet season.
Rain, or copious watering outdoors could help by washing away eggs, etc.
Someone mentioned regular fogging. For fogging to be of any use, it has to be done regularly. One-off fogging never achieves much in the long run.
The chemicals actually used in fogging should be carefully considered, too. Many are straight pesticides/insecticides in a kerosene base. Nasty shit.
Flea bombs aren't much better and obviously you can't flea bomb your garden. Agent Orange might do the trick but I don't know where you can buy it these days. Bad joke (sorry).
Neem, citronella tea-tree oils, etc, might be of some help.
I'd also be looking at buying some high-quality mosquito netting.
:grief: