Fence. Period.
I have close to 100 chickens and work hard to keep them where they belong as I know just how destructive they can be. You only have a few birds. They don't really need all that much room. A small fenced area will not cost that much to build. Even if you can't afford to fence your whole yard, the chickens will be totally fine with less. Many, many chickens in the world live in their runs full-time with no ill effects. Free range is nice, but not necessary.
For temporary fencing, you can try T-posts and a roll of plastic snow fence. Cheap and effective, but ugly. You will need to clip wings to keep them in it, though.