its a bit silly. There is no way of restricting the availability of knives, none that are remotely practical at least.
Its just something we have to accept - we can control the availability of dedicated-weapons that serve no other purpose then being a weapon (guns etc), but we can't control the availability of every-day objects that could be used as a weapon as well as their primary use (bats, knives, cars even)
If you can't solve the problem by dealing with the availability, then you must solve it by dealing with the causes of knife crime. Start with single-parent families, and the lack of dads in poor communities.. then work on to re-creating proper local communities, and keep going from there.