I run a guest house. how do I proceed with an invoice ( no paymment received) when the guest has cancelled his stay?

I got you covered in this situation, @matthiasherren.

When a guest canceled his reservation at your guesthouse, you can void the invoice made since you haven't received a payment. Voiding an invoice cancels the particular transaction. It will keep the invoice number and list it in reports but changes the amounts to zero.

To do that:

1. Go to the Sales menu at the left pane.
2. Find and open the invoice. 
3. If you want to include the void date, add a note to the Memo or Message section.
4. Select More in the footer.
5. Hit on Void. 
6. Click on Yes to confirm. 
7. Tap on OK on the confirmation message.

You can also visit this link attached to give you more details about voiding or deleting other transactions in QuickBooks Online: Learn how to void or delete invoices, expenses, and other transactions.

Know that I'm just a reply away if there's anything else that you need. I'd be happy to help. Have a nice day!