Unless you're trying to operate organically, now you know you have issues with horn buds and flies, consider applying crovect or similar to the head of vulnerable lambs as a preventative in future.
In addition :
- use Red Fly traps
- avoid fly havens for lambs during fly season (ie., put them not near natural still water, somewhere open and windswept, ideally no stock fencing or anything they can rub on and damage horn buds)
- watch for cracks in horns and apply stockholm tar before the flies get on it
You can use things like Citronella as fly repellents, but unless you've limited the fly population as above, that won't be nearly enough on its own.