His chakra is in all the curse seal he implanted on his victims. Therefore in order to completely seal or get rid of orochimaru, you have to hunt down and seal/kill all the people with the curse seal and there are loads. Sasuke after killing orochimaru, went to the hideout and released a whole host of them into the wild.
Its near impossible to track them all down and seal/kill them therefore orochimaru is always capable of coming back, through this means