Somebody's really going to have to go out of their way to email a patch — they might even read the `CONTRIBUTING` document which says "Thanks but no!".
The Github Pull Request interface is an attractive nuisance, like an unfenced swimming pool. You can't keep people out with warning signs, they won't read them. Put up a fence, though, and even the obtuse will get discouraged.
You are doomed, then. They can always sent a (unrequested) email to you, Cc'd to a public mailing list. The PR interface is just another equivalent way for people to send messages to you, like email.