1) Something that is made to be thrown away, single-use.
2) Someone who people will use. They normally don't like being disposable but they don't know how not to be. The term came from the first definition.
1) Did you throw away your disposable camera after you got it developed?
2) Breanna's really cute and nice, but for some reason people think she's disposable, so she doesn't have many real friends.