Why would you use neem oil? Neem is more geared for pest problems, specifically insect pests, not a fungus problem. I would lean more towards a fungicide like Clearys or even a generic soap to help bulk up the waxy layer on your plants leafs, if a plant has really thick cuticles or a wax layer is makes it much more difficult for a germ tube from a germinating spore to penetrate into the plant. Eventually the rust will go away with some fungicide application, whatever product you chose to use, just remember, follow the directions very carefully.
Nepenthes - hail to royalty
I bought it because the guy at the nursery recommended it. It is what he recommended I use for rust.