How do you get a Ghost out of somebody?

[Vahktang]

Some ghosts have the Malevolence ability (MM, pg 118), which is similiar to Magic Jar (PHB, pg 250). It's a supernatural ability, so it can't be dispel magiced (DMG, pg 289).
So, how do you get the ghost out of the character, once it's in?
If you can, stay with the core books (mentioned above).

[Toliudar]

In core:

This seems well within the power of a Wish/Miracle. For the lower cost option: give the affected person a ring with a continuous Protection from Evil effect. It doesn't force the ghost out, but it prevents it from exerting control.

Outside of core:

The first level ability of a Sacred Exorcist (Complete Divine) does what you're looking for.

[Twilightwyrm]

The 13th level ability of a Spirit Shaman also accomplishes this. AS does the previously mentioned Sacred Exorcist ability.

While it is not (partially) in the RAW, a thematically appropriate method would be a cleric using Turn Undead. Doing so would either cause the spirit to flee, exiting the possesses creature (not strictly necessary within the RAW) or be outright destroyed (I believe this is actually in the RAW, since the turning would destroy the ghost, but presumably leave the host unharmed)

[peacenlove]

Imprison Possesor and Impotent Possessor are 2 5th and 6th level spells in BoVD that render a possesing force unable to leave on its own and unable to affect the host in any way. They are Permanent and dismissible abjurations so just use a helpless body, trap 100+ spirits and dismiss the spells when those pesky adventures come close.