Anyone know how to delete unused textures?
What exactly do you mean? Delete from where, and for what reason?
I don't use DS, but in the apps I do use (Bryce and Poser) I find that saving the scene, closing the app and then re-opening it before rendering will remove all the unneccesary textures, and thus make the render go quicker.
In RC2 it removes the textures from memory if you select New Scene or just reload the scene. Make sure you save it at the right point!
In the past (DS3) Studio did not always delete textures from objects deleted from a scene. in some instances when you go to save the application will go looking for the parent of an orphaned texture that the session still thinks is being used, this causes a longer save time. Adamr001 had a DS3 script to clear the undo buffer which removed those textures but I'm not sure a DS4 version is out, or if DS4 fixed this issue. I'm going to DL DS4 RC3 this week and see. In the past to delete texture from the list it sometimes took a while before it would disappear if at all but you shouldn't have to save, new, reload if that's still the case.