The answer of Erwin is correct for someone located in Belgium like you. However certain countries allow to delete invoices, and there is a module for that called account_cancel. If you install this module, you will be able to go to a journal and check a checkbox that will allow you to cancel and delete invoices pertaining to this journal.

I'm not aware of a possibility to just delete invoices. That is probably a good thing since accounting-wise every entry should be traceable.

So just return all delivery orders and create credits for them. This is also a good exercise. ;-) If you have too many fake invoices it may be simpler to start a new database and keep the current one for testing.