I said "and you could add something like forced sterilization for people not affording to pay the "extra baby tax" after they've made one", but you could easily add jail time in a society where loosing money is not a good enough deterrent (though when the social services will take someones house and car away and put their children in foster homes because they won't be able to pay the "extra baby tax", I think the deterrent would be enough...)

I seem to recall some people paying people for sterilization. That would seem to solve the problem.

NOTE: The only eugenics I am for are those based on voluntary gene replacement.