So I ask, what happened to the trust anymore?
If something doesn't go your way, and sounds too "Good" to be true, or you never did it before yourself, it's complete bullshit?
So, you assert that you found a security issue with some site and they refuse to acknowledge it?
So, ... give them a reasonable time to fix it, then publish an alert here (with some info of where the vulnerability lies, or how the exploit works), if it doesn't get fixed.