What's the best way to train a cat to not get into the window sill?

Our cats get into the window sills and chew up the blinds and the curtains. This is not only annoying but I'm worried about what they might ingest - plastic and string are not a good additive to the feline diet! We live in an apartment, so I can't take the blinds down, they don't belong to me, they belong to the company that owns our building. We keep the blinds drawn because we don't live in a very safe neighborhood. I have tied up the strings to the blinds so they can't get tangled in them, but I want a real solution to keeping them out of the windows.