You can try
this.parent.instanceManager.removeInstance(this.parent.index);
But you need the following settings..
1) The parent subform should be flowed.
2) The subform that would like to destroy also should be flowed.
3) In the binding tab, check the Repeat subform for each data item.
4) Set the Mimimum count to ZERO.
5) Set the initial count to ONE.
6) Save the form as Dynamic XML form.
Place a button in the above Subform and paste the code in the Click event of the button.
Thanks
Srini