Are they in lots of different folders, Henk? If so.....
I don't want to publicise this, but I trust you will proceed with great caution.... Whether you are in a collection or folder, the undocumented Alt+Ctrl+Shift+Delete on PC or Alt+Cmd+Shift+Backspace on Mac deletes without any confirmation or other precaution. So if you are sure you aren't making a mistake, and everything is backed up just in case, it might help you.
John
PS Same Henk?