How can I prevent users with the same email from registering multiple guest accounts? We have cases where they are reusing coupon codes for the same email, even after checkout using the guest checkout. This is a Sitebuilder site using Kilimanjaro.

Have you looked at the native duplicate email management feature (Setup > Enable features > Web presence)? If that doesn't meet your needs, you'll probably need to replace the standard registration function with a custom SSP.

Yes, and thats what i was thinking but was looking to see if anyone has had to do this before.

We have multiple brands, so had to go the custom route to allow multiple registrations across brands, but stop multiple registration within a brand. It's basically just an SSP page to replace the login/register touchpoint and an .ss service file to handle login/register requests. Using the commerce API in SS1 is pretty simple as the methods are all pre-defined. Alternatively, you could do a hackier version just using jquery or similar to intercept the standard register form submission, looking up the email with an .ss file or suitelet before stopping/allowing it to submit depending on whether it's a duplicate.