How to free memory of an associative array

[Mark Isaacson via Digitalmars-d-learn]

How can I free the memory used by an associative array?

I need to be able to reuse the same array, but set it to an empty
state and free up the memory it used previously.

I do not believe that setting the associative array to null is
sufficient to free the memory, as it is possible that someone
still has a reference to an element inside, and so the garbage
collector must be conservative.

Thanks in advance!