You can delete messages using modem AT command and executing it over ROS CLI command /interface/lte/at-chat lte1 input=“<SOME_AT_COMMAND>” <wait=yes>, see more in ROS LTE manual how to use at-chat. AT commands depends on modem used by device, for your modem (EG18-EA) see this manual.
For eg. this performed AT command should delete all messages from SIM:
/interface/lte/at-chat lte1 input="AT+CMGD=,4"
To list all stored messages:
/interface/lte/at-chat lte1 input="AT+CMGL=4" wait=yes
This is not improved in ROS 7.14 for sending SMS messages with:
*) modem - fixed SMS removal (introduced in v7.13);
Tested now with 7.14, sent messages are stored in SIM and needs to be deleted with AT command for cleanup. To avoid this case ROS will need to implement sent messages list with possibility to remove them like for received in /tool/sms/inbox or delete from SIM when message is sent.