R86: To add to the reply from Chronos: The text of shield states, "Wielding a shield increases your Armor Class by 2," so a character could simply choose to not wield the shield and thereby lower their AC by 2.
The RAW state that an attack is made against a creatures AC, and as far as I could tell there's no allowance for a creature simply allowing themselves to be hit. So "check with your DM" is going to be the answer to that part of your question.