Welcome to BYC!
Are they first-generation Isa's from a hatchery, or are they second generation?
It's normal for every breed to sometimes have a very skewed ratio, and 55% of the chicks that hatch are boys, even under normal circumstances. And no, you can't do anything to stop it. Your case is pretty bad, but I know a man who hatched eleven roosters and one hen out of a dozen eggs.
Right now, I have twenty-nine chicks, sixteen of which are definitely cockerels. Bottom line: If you want to avoid roosters, don't incubate.