How do you delete or un-install a game from my ps3?

I'm not sure whether or not you can, but I only have 40 GB left on my ps3 out of 111. I used to have a total of 16 games now I only have 6 or 7 becuse I beat the rest and sold them back to Gamestop. Is there a way I can remove those game installments and restore those lost GB's?