If people continue to do it illegally, everyone involved should be prosecuted. In the same way we would prosecute any other criminal. I don't think it's right to say that there's not point making laws as people will just break them anyway! I do understand that is the argument for things like drug taking, but for things that permanently harm others I can't see that we can just shrug our shoulders and let them get on with it.
Sometimes laws are shit and inhumane. America's abortion laws are shit and inhumane. But just because another country is passing shit and inhumane laws, it doesn't mean that we should just let people continue mutilating their children. Quite the opposite; we should work harder to make sure our laws are better. If the whole of the west gives up and just lets people do whatever the fuck they want as it's easier than trying to stop them, I can't see it ending well. I personally don't feel that a country that allows people to butcher their children because they don't feel capable of stopping them, is on the right path.
I ask again... if you thought your son had problems with his spleen and all the doctors - NHS and private - said he didn't, would have it ripped out regardless at some back alley clinic? That's what you are claiming would happen if elective circumcision was banned, and you are arguing that because there are parents who would behave like this, we should just let them get on with it. With maybe a bit of extra regulation about cleaning the scalpel.