Ok, how about this: Make it so demons inherit the traits of dead demons. You know, like how human kids inherit the traits of their parents. Or is the wishing limited by arbitrary rules that prevent trivial solutions?
! The demons actually already have solution to stay in their humanoid form without the need to eat humans, this solution is most likely came from another wishing that someone made in the past, if you read the manga, you probably know who and what this is.
Some demons also like to hunt humans just for fun, like sport. That's why Emma came to the solution of separating the two world forever. Also, thinking about it, if demons trait are changed, it is kinda the same as erasing all the current demons and replacing them with a whole new creatures with different traits.