If I have durable power of attorney for my father, can I stop him from giving money to a relative who is taking advantage of him?

My father has been giving quite a substantial amount of money weekly to a grandaughter who he says is not able to work. We as an extended family know that is not entirely true. I have DPOA for my Father. What steps can I take to stop this? My Father lives entirely on his S.S. He seems competant in other areas, yet beleives the lies she tells him.