Yes, you have the possibility to delete a message that has been already commanded and added to schedule, but since the process is a bit tedious in terms of configuration, you will have to download the PDF file offered by the developer. You will find all the instructions along with each step and image detailed about this process. Basically, a message that has already been commanded can be canceled with the help of another message. Simply send another message and it will overwrite the current one that you want it gone.
