It is not illegal to shut down a website for non-payment and stop all services, including mail forwarding.
You missed the point. Yes, you can shut them down, you cannot keep accepting their email and hold it hostage, it MUST be rejected or you MUST give it to them. Like I said yesterday, if you don't pay your rent, your landlord can't hold up your postal mail. You can get locked out of your apartment, they can put a note on your mailbox MOVED so the post office doesn't deliver (like REJECTED), but they can't hold your mail hostage, and you can't do that to email either.
So yes, you can stop ALL services. Jale