If you're not opposed to mods, you can delete the diplomas using my destroyobjectcheat Mod. It can be useful in other situations as well, e.g. for deleting trash plants, pet poop, stinky leaf piles, paintings in open areas blocking the easels, fires that cannot be distinguished because of a bug, struck festivals objects, objects like books, computers, bikes that have been left somewere in the open area of a neighbourhood, ...
https://deichschafblog.de/bienchen/sonstige-mods-misc-mods/ (MM-26 destroyobjectcheat)