I'm going to send back my parents to their place when they came for an unexpected visit
Well, did your parents wear out their stay already? How can you just send them back now?
I need to focus on my thesis, I've had enough. I don't want my new room to get messed up.
You were busy with your thesis which is important to get done, but your parents really missed you. They should have let you know they were coming, otherwise people will be caught off guard. Be polite yet firm, and tell them you would prefer them to come over another day because of your studies.
I just don't have the time for them.
That's understandable. A busy college student don't have a lot of free time. But you should still try to make time for family.