Is it permissible to secretly take money from one's father to pay one's Zakat, which he does not observe, after advising him and his refusal to listen?

Zakat is a great pillar of Islam, and whoever withholds it commits a grave sin. Zakat is not valid except with the intention of its owner, for the Prophet, peace and blessings be upon him, said: "Actions are but by intentions." If one donates money and does not intend it as Zakat, it is not permissible to count it as such later. Similarly, it is not permissible to pay Zakat on behalf of someone else except with their permission, because permission implies the intention of paying Zakat. Therefore, your taking money from your father and paying Zakat on his behalf without his permission does not discharge his obligation, and you must continue to advise him.