No sent messages can ever be permanently deleted in SComm. However, they can be removed to the 'Trash' folder. To do so, select the messages in the 'Sent Items', next click on the 'Remove Message' button.
This will remove the messages from 'Sent Items' and store to 'Trash'. If needed, you can restore your messages from 'Trash' to your 'Sent Items'. You may also move your messages to a separate folder that you have created.