I had this very same problem. I moved around 30 graves into a sims inventory, and had them all placed in the park. Now I have around 5 to 7 ghosts all in my house eating my leftovers and sleeping in the beds.
The way I got rid of them is to actually remove all the leftovers from the fridge. Then the ghost left in search of food. Trouble is I cannot move all the graves from the park now and they seem to be stuck there. Oh well, i s'pose its a bit of fun eh? --- I hate people who take drugs......Customs men for example.