Not sure why you say "remove DVD from drive". The ISO is normally mounted from the hard drive, and actually this is one of the strengths of these tools. And I am not sure why you have ISO files on DVD, as normally you burn the DVD in it's native format not the individual ISO files. And of course if you remove the media with the mounted ISO files, the tools will have errors, as they cannot find the target.
In Daemon Tools, you can click (regular left click) on the icon in the systray, and you see 2 lines, the first is unmount all drives.