If you want to cancel a bank transaction that was included in a statement that has been reconciled, then you must unreconcile the bank statement first. 

Unreconciling a bank statement will return the items listed to the list of bank entries waiting to be reconciled. 

Important! Unreconciling a statement will cause all subsequent reconciliations to be automatically unreconciled. So if in June you unreconcile a statement from April, then statements from May and June will also be unreconciled.