I am assuming you are talking about lightroom since you failed to mention what program or its version or operating system for that matter.
- In lightroom select all the pictures for that folder/directory and save them ctrl-s or cmd-s This will save the metadata for those files to the hard drive.
- Mark all of those files with a flag, number or other means just to ID the ones you want to keep (I am not sure if this flag stores in the metadata or is just a database feature, so save for this step as a precaution)
- Now you can sync that folder/directory (This will bring back in those you discarded, which was why you needed to mark the other images and save the metadata as syncing can clear any edits if you don't save the metadata)
- Now you can use the search feature to select all images in that folder/directory that are not marked (It maybe a good time to pan through the images and make sure there are none you want to keep)
- This time instead of discarding you can delete those you don't want.