We're unable to delete a reconciliation report in the regular versions of QuickBooks Online. Though, we can manually undo each transaction in the report. I'll guide you how.
From the Gear icon, select Chart of Accounts.
Find the account and then click View Register.
Locate the transactions you want to edit.
In the reconcile status column (indicated by a checkmark ✓ ), repeatedly click on the box until it changes from "R" to blank.