When my father talks to someone, he will not let them go. He continues to talk and talk (not making much sense) even when they say they need to get going. He gets very angry if we tell him it is time to go or that the other person needs to get going. His response is often, "You can't tell me what to do."
Some how signal the person Dad is talking to "they" need to cut off the conversation. If ur visiting, the host needs to say "Well it be a nice visit but we have another engagement" or "we have an early morning, so time for us to go to bed".