illegal residence and occupation of private premise,by strangers,need help..

Hi all the worst that I hate is my grandfather died a few years back and some stranger(midlle age)woman took the opportunity to move into his premises/house disregarding the rightful owners and make it her living quarters.
And i must although greek not from kithira but rather a island close by.
My grandfathers house is located in Agios Hlias .
And I believe in order to throw her one must take legal action.
Does anyone know what else ,one should in these circumstances or experienced similar ?