To add an item to list use XmListAddItem() with 3 arguments:
Another function XmListAddItemUnselected() has exactly the same syntax as above. This function will guarantee that an item is not selected when it is added. This is not always the case with the XmListAddItem().
To remove a single item from a List use XmListDeleteItem(Widget, XmString).
To remove a number of items use XmListDeleteItems(Widget, XmString*).
If you know the position of item(s) you could use: XmListDeletePos(Widget, int) or XmListDeleteItemsPos(Widget, int num, int pos).
The latter deletes num items starting at position pos.
To delete all items form a list use XmListDeleteAllItems(Widget)