Vaseline, cooking oil, pine tar, olive oil, anything that will smother them and drown them. All it really takes is one or two good soakings, the entire foot and leg being covered all the way up to the thigh feathers; I prefer to use pine tar (stockholm tar) myself because it heals and disinfects at the same time, but some people would rather use harmful chemicals (totally unnecessary, even plain oil will do the job).
There is a genetic predisposition to them, if the animals get them badly and often, then they are susceptible. That susceptibility goes hand in hand with other weaknesses.
Diet can be to blame to some degree, adding garlic and anti-vermin herbs to the diet regularly can fix it, but culling out such weak lines is a better long-term idea if you breed them. Resistant birds just never get them regardless of living with heavily infected birds.