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

[aliak]

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!