Well guineas are good mosquito repellent, but thats not a quick solve. Try making ALL water sources not near their coop so their less attracted, perhaps find some sort of natural mosquito repellent (yarrow flowers work great) and rub/ spray it over the coop.
Well, yarrow is a flower, looks like this
But I;m not sure of what types of natural mosquito repellents are callled. I can look if you want
(actually I'm looking right now)