Is there a way to disable copying of an alias this'd member? - trying to make a NotNull type

[aliak via Digitalmars-d-learn]

Basically, I want this to fail:

auto notNull(T, Args...)(Args args) {
    return NotNull!T(new T(args));
}
struct NotNull(T) {
  private T _value;
  @property ref inout(T) value() inout { return this._value; }
  alias value this;
  //disable opAssign to null as well
}

class C {}
void func(ref C t) {
  t = null;
}

auto a = notNull!C;
func(a); // i want a compile error here

Any ideas that don't involve disabling copying or making the 
property non-ref?

Full example here: https://run.dlang.io/is/ubOwkd

Thanks!