How to reset the value of a V8 variable?

Question

C++ / C#

How to set value for V8 variable to undefined ?
Let's say there is a global variable:
Persistent<Function> callback;
How to set it to undefined so that memory is also freed (although I suspect the GC will do this itself).

Answer 1

delete callback;

Answer 2

Stupidly
obj->Set(v8::String::New("propname"), v8::Undefined());
doesn't roll?
I re-read the question:
In general, Persistent has a method Dispose
callback.Dispose()