Direct them to go into the light. Tell them that anything they were hoping to get/fix/prevent/control can only be handled, or can be handled better by them going into the light. That they cannot understand the meaning of their life's purpose until they go into the light. They have no power or wisdom until they go into the light.

When dealing with a ghost parent or grandparent who is possessing their descendants, point out to them how their actions are causing pain and harm to the vessel, and ask if that is what love would do? They had their time, now the descendants have their own lives to live and it is not fair to try to cling to life by stealing the life of others.

When dealing with a dead parent who wants to watch over their living children, tell them that is if is as Goddess Wills, they will be allowed to return, able to do a better job of it if they go into the light first and get the after death orientation to discover the purpose and meaning of all the events in their own lives, and become empowered. No time will pass in this reality, while they are gone. You may see them go, then reappear a moment later, much changed: radiant and selfless.

   Whatever motive they have for whatever they are doing, tell them that whatever they want or need is in the light.
   Tell them that there is a wonderful welcome home party, waiting for them in the light.
   People who love them are waiting for them in the light.
    Repeat as needed.
   Accept no other solutions, suggestions or excuses and answer all of their complaints with a suggestion to go into the light. Then ask your angels and their angels to escort them into the light, and thank the angels for doing so.

Very rarely, I find ghosts who will not listen to me, for whatever reason. Simply ask Goddess/your guardian angels or theirs, to bring someone from the light whom they will respect and listen to, to persuade them of the truth of what you are saying.

I find this is often the case when dealing with some Native ancestors, or ghosts of other cultures who practiced ancestor worship. The ghosts are convinced that their proper place is to nag the living into adhering to the old ways... which can be devastating to the descendants, if the old ways are gone the way of the buffalo. If that is their place, then like parental ghosts, they may be allowed back once they have gotten the after-death orientation... and they will be stronger and wiser, better able to serve the descendants.