There's no way to delete a transaction that has been paid for in full online—which in-turn allows you to delete the appointment.
If you've refunded an appointment that was paid for via Online Bookings, the money will be automatically refunded back to the client's original payment method through the Stripe integration.
Because this a 'live' payment method, the record of the transaction has to stay. Otherwise, transactions in your Stripe account won't match what is in Simple Salon.
One way to deal with the appointment is to use the double book feature to place the appointment in a second column.
Once the second column loads, you can then pop the appointment into the second column, so the normal column is empty at that time. This would ensure that the space is still available for bookings.
The following article will show you how:
Another option is moving the appointment to a past date, so you can book new appointments in it's spot.