I have had different reactions.
If it is a mild annoyance, (a person engaging in SOCIAL jabbering) I can nod and "mhmm" my agreement and not invest any real emotion in the matter.
If it is an intermediate annoyance, (a person engaging in ACADEMIC jabbering) it depends on how close I am to them. If they are across the room I close my eyes and pray that they will shut up soon. If they are right next to me, saying something like "Well, I know that this is a religious studies class, and we are trying to be objective, but it is impossible to be objective, and I was raised christian, and I was taught that all of these religions were wrong and...[insert opinions and ignorance here]...and so YEAH!"
THEN I grip my pencil while leaning back, and I contemplate thrusting it between a pair of vertebrae rendering them paralyzed, incapable of speech or controlled bowel movements.
If it is a massive annoyance, (Someone accusing me of something, then when I try to put space between them and I, following me to inform me that I am a generally bad person) Then I smile at them until they stop, I nod, say "you are absolutely correct. I do not know what I was thinking when I disagreed with you earlier, please forgive my unfounded arrogance," and wait for them to walk away. The last time I had to do this I moved into a defensive fighting stance because I thought she might lunge at me. The time before that, she was about to cry and was far enough away that I could avoid assault.