How do you deal with a person who won't stop talking to you?

When I used to visit my sister at college, I took the bus and one time I had this guy who just started talking to me. I had my headphones on and was looking out the window almost the time. I even went to the bathroom and he kept talking to me when I got back. I tried to be polite, but it was for the whole 3 hours, and I was wondering how you’ve dealt with people like that. Maybe some advice for next time? How long should you carry on a conversation that you’re not interested in?