Thank you for the Best Answer award however I think you'll like this answer even more. We've just added functionality to the site that will allow you to edit or remove open house times and add new ones to your listings yourself so there's no longer a need to contact our Customer Service Team for assistance.
Just access the My Listings section of your account and select the 'Open house info' link and make your updates. They'll then be live on the site within 24 to 36 hours.
I hope you find this useful - please let us know if you have any feedback or questions about it.
Have a great weekend!