Quite simply, if you don't want a caller to be able to use an instance of your class as if it were an instance of the base class, you shouldn't have that inheritance relationship to start with - it breaks the Liskov Substitution Principle.
I strongly suspect that Figures
should use composition instead of inheritance - so it would have a private field of List(Of Figure)
instead of deriving from it, and you'd expose whichever operations you want to, and only those operations. Most operations could probably just delegate to the list.