bin/console media_library:cache:clear

Will delete the /src/Frontend/Files/Cache/media_library_* folders. Which means that all frontend thumbnails will be generated next time on load.

By default the backend thumbnails will remain cached. If you also want to remove the generated backend thumbnails you need to append the --all option.

bin/console media_library:delete:items

Will delete all MediaItems (and its source, backend and frontend generated thumbnails) that are not in use.

If you also want to remove the items that are in use you can do so at your own risk by appending the --all option.