Mmm... there are not many chickens about which are completely free from lice. A few do no harm but if you've noticed them then there are probably too many.
Spraying birds is very ineffective as feathers are excellent insulation.
You really need to find out what species of lice you are dealing with by googling. Some live only on the feathers, some hide between the fluffy feathers (filoplumes) and are bloodsuckers and a few only crawl on the chooks at night.
Dunking the chooks in a bucket of lukewarm lice preparation diluted to the proper recommendations ON A WARM DAY is a quick way. Completely submerge the fowl except the head (hold this cause if they struggle they may drown themselves).
If you have biting lice they can be shifted with Ivermectin injectable formula for goats/sheep and use that as an "spot-on" on the BARE skin at the back of the neck. Work out how many drops per KG of live weight from the directions or get it from a vet, they have to supply it with application dose but may charge a few quid extra. This will NOT shift those long narrow lice that you can see neatly nestled between the barbules - those eat feathers/dust and won't come in contact with the bird's body fluids.
If you (the chooks lol) have the type of lice that live in the nooks and crannies and attack the birds at night for their fill of blood you need to ofcourse spray the shelter with a strong disinfectant. Actually paint it on the perches and when dry put vasiline on the ends where it is jointed to the house so any insect crawling towards your egg producers will get stuck.