@Chen, Cherie Thanks for posting your question in Microsoft Q&A, apologize for any inconvenience caused on this.
- If you want to delete those dead letter messages from the service queue change the Queue to Receive Mode, select ReceiveAndDelete option and specify the maximum number of messages you want to receive and delete. This helps you to delete the dead letter message without passing any session Id's.
- Alternatively, if you want to delete all the dead-letter messages without specifying the maximum number of messages to be deleted then you can perform the same operation using Service bus explorer from local machine. Download the ServiceBus explorer exe from this GitHub Repo
Hope this helps, let me know if you have any further questions on this.