How do you get a ghost out of a possessed person?

Human entities do not typically "possess" living humans - possession is generally confined to non-human entities, those which have never had human form or walked the Earth. They are also more adept at possessing a person who may have lost their faith, or were weak psychologically and unprepared for an attack.

Possession can also occur in those who are knowledgeable but are careless. If a person is indeed possessed, you must first find a person or group knowledgeable in possession, and with experience in forcing the entity from the person inhabited. Modern medicine does not recognize such things (at least in the U.S. - we're too "modern" for that..), and such people aren't easily found.

A good start is to contact any local Paranormal Investigation groups, or see if any local Colleges have a Paranormal program in place - many do. From there, they can usually point you in the right direction. Experienced Demonologists are plugged into a network of people who have the proper experience to deal with this type of problem.

If you believe someone may in fact be possessed, be extremely careful - such individuals are acting under coercion and don't realize it, even if they're close to you. There is one case that comes to mind where an experienced investigator came across a site that was protected by Satanic spells, and fell under the possession of a guarding non-human entity. She came close to killing her own son before she was freed.

The TAPS site has a good list of Demonology subjects, including a "who's who". Check the link below. You can also post on their site to ask for more information on the subject.