When you´re in the booking window on each of the tabs at the bottom right there is the delete button, when you click delete the are you sure yes or no box appears at the top of the window, click yes to delete it.
If you don´t see the delete button it maybe disallowed in your account settings especially if youre using a sub account.
If youre using the master account the setting is in account management then manage account then it is the top setting on that window.
If its a sub account your using its in account management .....manage account on the sub account and then in the second set of settings called account settings.