We got something from the vets - I think it was called hexocil - it was a dog and cat shampoo but it worked a treat on our donkey who was infested with them when we got him.
Powders didn't touch them so don't even waste your money!
i get some stuff called pour on, it is for cattle put a small amount at the base of the mane and the top of the tail, it cost about 36euro over here but it has lasted me for about 3 years and i use it again in the summer as fly repellant.
If you can get hold of some i would seriously recommend it