Solution is quite simple.  Wills often have conditional clauses in them.  

The money simply gets held in a trust until they can demonstrate to your Executor that they've passed an accrediated University or College "Finance 101" or "Investing 101"  class.  Heck, if it's a lot of money, make them pass the CFA for it!    

 If that's the only thing standing between them and a bunch of free money, they'll learn.