You don't have to go 'a=null'. You just have to eliminate all references. In the case of adding it to a list, it is sufficient to remove the object from the list, if it is the last reference to the object in question, the object is automatically garbage collected next time the GC is invoked.
Local myList:TList = New TList myList.addLast(Thing.Create("whatever")) ' the object is now added to the List collection. myList.RemoveFirst() ' remove the first object from the list - it will be garbage collected if it is orphaned.