Can't Set Available Quantity to 0?
Occasionally, merchants have encountered a glitch on the Gateway platform where the Groupon Available Quantity remains set to “1”, even after the Merchant Available Quantity is set to “0”.
Fixing the Issue
Look up the product from the "Products" page on Gateway.
Set the Merchant Available QTY to any number HIGHER than 1 (don't worry, this will only be for a second!)
Refresh your browser.
Set the Merchant Available QTY to "0".
Click "Update" again.
Refresh your browser, and the product should now be unavailable.