If you are a group administrator with the necessary permissions, you simply need go to the Group's gallery or favourites tab and enter Edit Mode, by selecting the "Edit Page" button near the top right of the page.
An "Edit Your Gallery" banner will then appear near the top of the page. You can then click the Edit Pencil icon that appears next to each deviation when you hover over it. There will be an option to "Remove from This Folder."
If you remove all instances of the deviation from every folder it is displayed in, the image will be completely removed from the Group's gallery and favs. Once you have finished editing, you can click the blue "Done" button near the top of the page to set the changes.