What do you mean by that? :p It's pretty commonly accepted that, once you've taken ownership of a lost soul, you control that player's actions after that point. Since all drops lead to a lost soul being created, there's nothing you can really do to prevent another player from claiming that character and making up whatever new stories they want to about that character.
Unfortunately once you hit the drop button, you lose a say over that character :p Your character is entirely still alive and just hanging about. They can say they died all they want, but reality says otherwise heh.
Subject to reasonable complaints of harassment, yeah Ulrike's right, you can do what you wish.
AFAIK it's not enforced by anyone, but taking a lost soul and messing up its prior RP is in poor form in any event - so even if you CAN get away with doing it, don't be unreasonably antagonistic or taunting in playing with another player's former character.