Suggestion: Free Nation Flag (and limit buying them all)
First off, this idea will not lock out the buying of national flags. You can buy them all, but this is just a way to lessen the psychological need to buy them all, as if some achievement you have to do.
Basicly, you declare what nationality you are, maybe able to declare two for duel citizenship, and you get a free, or discounted price on the national flag(s) that's available. You then remove the spam of nantional flags from the decorations menu, and put it in the same profile menu from where you got your first flags from. For the current price, but out of view of the easier to get at decorations store, you can buy more flags
The principal is 'out of sight, out of mind', by removing the flags from the normal decoration store, and putting them one or two menus deeper, in-game. You still get your flag or flags for your nationality, and for support for other nations, for whatever reason, but should reduce the spam of flags for just the reason that they are there. This will also reduce the spam of the store, and sort national flags in a better menu, and possibly allow for more flags to be added in-game.
National Flag to where you call home
I think they should just ask what nationality you are on the game and once entered either supply or allow purchase of national flag of only the country you call home , saves any hassle of people being able to buy all the flags when they don't need them. This way everyone is happy they have their nations flag flying proudly in their base. Come on supercell, I mean the Australia flag is there, New Zealand is definitely better than Australia:)