Engaging in even the shortest exchange will waste time as they are trained with an answer for everything.
Open the door, hand back the books, say you don't want any further contact thankyou, and close the door.
Anyone calling again you should simply say to put you down as a do not call, close the door.
Refuse to answer any question on anything.
They will get over it so quickly it will amaze you. Even after 4 years you are not really considered a true friend. Now you are just "Bird Food" and not worth worrying about.
If you carry on the study, continue to doubt, and maybe get baptized, you will be disfellowshipped for doubting, and then they will have a great time treating you like manure every time they see you in town.
HB