Bookem refers to deleted/cancelled invoices as voided invoices. Voiding/deleting invoices actually archives (hides) rather than removes them from existence entirely. It is standard practice to keep all invoices on record for auditing purposes to protect your business from errors or, at worst, someone who has access to your account that may be dishonest and fudges your invoicing records.
To void an invoice you first have to remove any allocated payments from it. This is done by opening the invoice and clicking on the payment(s) at the bottom of the invoice.
Once the payment has been deleted, reset the status to Void, Open or Uncollectable.