Is there a mechanic for discarding cards?
I didn't come across any. Just for players to add cards from their hand into the Collection Plate. This is curious since the FAQ on the back of the official rules states:
Official Rule Booklet FAQ wrote:
CAN I DISCARD SEAL CARDS OR "GOOD" CARDS?
Yes. if you are possessed this is a great strategy!
Unfortunately he doesn't cover how or when to do that anywhere in the rules.
What do you do with broken Haunting Cards? Bring them back to the home room and discard them? I think keeping them in the player who drew them's hand and discarding them into the collection plate is a good solution, because they don't take effect there in any case, and it obfuscates which cards were drawn from the home room draw pile.
I chose discarding them into the Home Room Discard Pile once the Haunting Card is broken and the player returns to the Home Room (see the cheat sheet, page two, under the Haunting Cards entry), as it's the simplest action. Also, the rulebook on page 11 mentions that:
Official Rule Booklet, Pg. 11 wrote:
Curse Cards and Haunting Cards may make their way into the Charm Circle through the Home Room Draw Pile.
So Curse and Haunting cards can make it into the Charm Circle from the 13-card Home Room Draw Pile, but the rules do not mention players being able to discard Haunting Cards there.