All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
There is no doubt that watching indecent films is prohibited and your father may reap grave consequences both in this life and in the Hereafter unless he repents and gives up this habit. Observing the daily prayers will help him relinquish that habit and you may remind him of the verse that reads (what means): {Indeed, prayer prohibits immorality and wrongdoing.}[Quran 29:45] It is inappropriate for someone who prays regularly to watch such programs.
On the other hand, you have to also be good to your father by offering him advice, exhortation and reminding him of the Last Day together with forbidding him from doing wrong acts. This should be done gently and leniently. If you can change his access password then you should do it so that he could not watch such things. However, if this would lead to quarrels and dispute then you should not do so.
We supplicate to Allaah the Almighty to guide you and your father.
And Allaah Knows best.