[flexcoders] Remove Object ref

[Matt]

In ActionScript if I create:

var obj:Object = new Object();
obj.first = 'One';
obj.second = 'Two';
obj.third = 'Three';

and then I want to remove 'second' from the Object how would I do so?
I've tried setting it to null:

obj.second = null;

However, that leaves it still existing if I iterate over it. Is there
any way to completely remove it?