You can delete your prenotification by checking the container and clicking on the ‘cancel container’ button at the bottom. The prenotification is then cancelled at the terminal in question. This triggers a message which still needs to be accepted by the terminal to get the status changed to ‘Cancelled’.
It is also possible to detach a prenotification from the visit. Select the container and click on ‘Detach container’ at the bottom. The container can then be found as a status request under the heading ‘Detached’.