As soon as we start debugging a CICS transaction, Mainframe Express runs the CESN transaction, even though no transaction is specified in the INITIAL TRANSID.   Is there a way to stop the CESN transaction from running?

To override the CESN trxn when debugging, you either need to specify a different trxn in the Initial Transaction Id on the Start Debugging screen or if you do not want to run a trxn at all then select the CICS tab.

Next, right click on the CICS System folder icon on the right hand side at the top of the tables list (PCT,FCT,PPT,SIT).  This will display a Properties link, select this and remove the CESN trxn code from Initial Transaction Id field on the Overrides tab.

Once you have done this, when you start debugging you will get a blank screen instead of the CESN trxn.