You can delete invoices from the Invoices page, accessible from the Sales menu. If there's a payment against the invoice, you'll need to delete the payment before deleting the invoice.
A deleted invoice is removed completely from MYOB.
To delete an invoice
Go to the Sales menu and choose Invoices. The Invoices page appears.
Click to open the invoice you want to delete. Need help finding an invoice?
At the confirmation message, click Delete. If you see a message about there being a payment against the invoice, you'll need to delete the payment before you can delete the invoice.