In the current release of Strategy Command Manager 9.x there is no wildcard character that can be used to delete all reports in a folder at once; however, this functionality can be achieved by using Java Procedures.
In this case, Strategy Command Manager 9.x has the sample procedure "Recursively_Delete_All_Reports_Under_A_Folder" for deleting all reports in a folder. This feature can be found in the Edit menu > Insert / Search Objects > Outlines > Procedures_Outlines > Sample_Procedures, as in the picture below:

NOTE: Please refer to the following technical notes for furthter information about Java Procedures in Strategy Command Manager.