Being rude and inconsiderate will also do much more harm than good. I know of no one who becomes more receptive to the truth when we are rude to them. Be kind and considerate. Many don't expect that type of treatment. They are trained to expect rude and inconsiderate people at the doors.
Nothing stands to be gained by being rude and inconsiderate.
It would also be a mistake to just say right off the bat that you aren't interested in what they have to say. In many cases what they have to say may be useful in teaching them. But put yourself in their shoes for a minute. They firmly believe that what they teach is truth. We want to be careful not to leave the impression we are not interested in truth.