To remove files from the concatenated files list, select the file or files to remove and press the Remove button or the Alt+E keys. You can select multiple files by holding the Ctrl key down while making your selection. You can select a group of files by clicking on the first file in the group, holding the Shift key down, and then clicking on the last file in the group.
To remove ALL the files in the list, press the Clear button or the Alt+C keys.