It'd help to know a bit more about why you want to delete the receipts. Ariba (depending on a few settings) will automatically create receipts in "Composing" status for some orders. This isn't (necessarily) a problem.
Now, Ariba will also let you close the order for receiving, and that will automatically move the receipt into Approved status. That wouldn't delete the receipt, per se, but you'll no longer see the order as though you need to receive it.
You could also change the receiving rules so that the receipts are not automatically created in this manner. There's a few different options for how receipts can be handled.
If you can give a little background on why you're looking to delete receipts, perhaps I can suggest a better alternative.