Invoices can not be deleted. What you can do is you can derecognize them so that there will be no dunning process triggered. This only works for invoices where we track the payment and where not an thrid party guarantees the money.
In order to do that click on Transactions, switch to the tab invoice and open the invoice you want to derecognize. On top you find the actoins to Derecognize, mark as paid, replace the invoice or download it.