Earlier quoted context omitted.
Why? It seems like you are escalating a war AGAINST your users. Most folks who use a disposable address are trying to make sure their main address doesn't become a fount of spam. If I continue to visit your site/service, then I will probably decide to "update" my address on my own for the convenience. Am I really being naive?
You are being naive -- it really depends on the service. For trivial online services, a disposable email might be fine, but for anything that costs money, you're doing your users a disservice by letting them register with an email address that they won't be checking (and which could potentially get hijacked, if someone knew the address).
If a service prevents me from signing up with whatever email address I want, I won't be using it. It's one thing to discourage weak passwords, as that's a common mistake for people that don't know any better. It's another thing altogether to deny signing up with something like this, as they clearly will know the ramifications of doing so.