The films were put there from another iTunes library. To remove only some of them you would need to sync to the same iTunes library, which you don't have. The previous owner really should have reset the device before getting rid of it.
Futhermore, to keep the films that are on there, you will never be able to sync with your iTunes library. Like all iOS devices, it can only be linked to one iTunes library and it is already associated with one (not yours). Trying to sync would ultimately wipe the content from the device and replace it with the content in your iTunes library.
That being said, it is best to just start fresh and set it up to your liking.
(From the main menu, select Settings > General> Reset Settings > Factory Restore)