Is it permissible for me to delete channels that display evil acts from a television set that I do not own?

If the evil is not removed by admonition, then it is obligatory for whoever is able to remove it by hand, so long as it does not lead to a greater evil. Removing it by hand involves removing the channels. If this does not lead to a greater harm, such as angering the parents, their reinstating the channels, and their boycotting you, then remove them. Otherwise, suffice with denouncing it by tongue and supplicating for them.