We had someone similar at our door a few weeks ago, but I can't remember the company. After that encounter, I looked online and decided that I was happy with our current arrangement with Reliance Home Comfort.
That's what I said to the two guys from National... "Thanks for coming by, but we will decline."
Apparently, that wasn't sufficient to send them away. As I was closing the door, he kept talking, "But we're a Canadian company..." I re-opened the storm door and said, "Do you even know who we rent our water heater from?" To which he answered, "It's either Reliance or [some other company that I can't remember>]." I retorted with, "See, you don't even know what we have. Thanks for your visit, but we will decline."
As I proceeded to close the door again, he had a stern facial expression and maintained eye contact with me. Think about it. That's highly unusual, and struck me as aggressive. At this point, I experienced a twinge of fear that these guys could possibly go beyond a sales call, and get nasty... I pictured them vandalizing my property or harassing my family.
I opened the storm door again and said, "OK, I don't want to be rude. I told you we're not interested. Thank you, and good luck."
I finally closed the door.
Try googling "national water heater". Most of the hits are warnings about scams.