If they're anything like mine, nothing will deter them from what they want to do. You'd have to actively prevent them by a barrier.
This is what I was thinking. you have plenty of space, it's not like you are dealing with a small suburban back yard. You may need some re-training for the birds, they're powerful creatures of habit. I'd set up a mulched area specifically for them, and I'd put up a temporary fence around the flowerbeds. Yep, it may not look the greatest, but you should only need it for a month or so before they get used to going somewhere else. Make that new area nice and tempting, perhaps even construct a temporary fence to keep them there for a week or so. Scratch thrown out there is a wonderful idea. You could also "seed" the area with some worms, etc until the natural insect population gets up and running.
I keep my chickens out of my gardens with a two foot tall green coated wire fence, I have never had on jump over it. It fairly cheap, bendable and easy to work with. We have put around many different beds.
It blends in pretty well, not sure why the chickens don't go over it but they don't.
