You can quickly check if the changes have been applied by re-opening the files in MetaImage. If the XMP:LensInfo is empty, it does mean that Flickr is using another tag you did not delete. You can use the inspect button in the window toolbar to list the remaining tags.
You can also send me a copy of the image at email@example.com. I will inspect the files manually to understand the source of the problem.
Thank you very much for the email with the attached picture. MetaImage displays the tag in the list because your image contains the Exif:Lens tag. The value corresponding to this tag is empty, and this is what is displayed by the app.
If you want to remove this tag, you can replace its value with a hyphen “-”. This a special value. MetaImage will remove the tag during the saving changes.
Let me know if it works,
Have a nice day,