I followed the following instructions:

Disabling VirtueMart 2.x.x PDF Invoices

To prevent customer from getting also VM2 native invoices, go to VirtueMart -> Configuration ->Checkout -> and make sure section "Default Order Status to send an invoice" is empty.

As soon as I make this section empty and save it the window closes and after reopening the settings go back into the previous setting. I cannot empty the section "default order status to send an invoice'.

I cannot figure out why?
I am using Version2.0.20b an Joomla 2.5.10

Hi Makl,
problem solved.
Make a new orderstatus label in VM and attach this one to check out settings. Now it works.