To delete a bank transaction just click Bank > View/Add button > Tick the box to the right of the transaction > Click the Delete button that appears in the top right hand corner.
Please be aware that deleting a transaction that has been previously reconciled will cause that and subsequent reconciliations to be incorrect. You will have to re-reconcile all reconciliations thereafter.
If a bank transaction is associated to an invoice or a purchase then you will need to simply click on the transaction. You will then be taken to the relevant invoice or purchase. In that screen you will need to click the Delete button next to the payment details in order to delete it.