First, make sure they aren't on a locked layer.
If they aren't....
I don't know if there's an option to turn on to make this work or not (look for somehting about "show kuid" in the settings) but you may be able to use the "grabber" tool (the one that selects and object in the assets list when you click on it in the world) to get the KUID. That tool will show the KUID as you hover over objects.
Take note of the KUID s.
Then find those assets in content manager, highlight them, and export them to cdps.
Then delete them.
Then delete missing assets.
If they are built in, you can open then for edit - and try to make them faulty (delete the mesh and submit) then again delete missing assets.
Once its cleared up, just reinstall the cdps or "revert to original" if it was built in.