Purge list of used files from dropdown boxes etc?

For several years now the method I have been using to clear out old IOS's etc from the drop down boxes within Parallels has worked just fine but now with v15 I can't clear "ppl-tools-mac.iso" (For example) from the dropdown box of the CD/DVD Rom source section.

The old post below advises to remove the line with the info on from the offending file but I used to simply delete the file and reboot, now since upgrading this no longer removes the entry as seen is the attached screenshot.
https://forum.parallels.com/threads...st-of-previously-referenced-resources.258074/



Can anyone help with this please?