Okay, Amster. Here's the deal. If you open a business, you will deal with shitty customers. Period. That is just what happens, and this is not some OH MY GOD WHAT HAS THIS WORLD COME TO?!?!?! moment. I'm sorry, but it isn't. Should OP have been civil? Yes. Absolutely, and I guarantee that nobody chiding garadget will argue that point. K? We on the same page?
Now. If you open a business, and your skin is so paper-thin that you just cannot stand it AND THREATEN TO BRICK SOMEONE'S PROPERTY over trash talk, you are no better than the person who was abusive to you, and you are a piss-poor businessperson. If I had ever once behaved this way to a customer during my stint in CS at a software company, I wouldn't have had time to flip the customer off before being kicked right the hell out the door, and my boss would've been right to do it. And I'll tell you what, the number of irate customers I dealt with were at this guy's level or worse, and I will tell you they made me so mad I could spit, but ya know what? Whenever somebody had someone they couldn't handle, they'd send 'em to me, and in about fifteen minutes they left happy as a clam. So if my impatient ass can pull that off, this dude has absolutely no excuse.
And on the topic of excuses, being a startup is no excuse. Being a small business is no excuse. If you cannot be arsed to at least feign basic human decency in the face of rudeness, sorry. You deserve whatever fallout you receive, and you should probably find something better to do with your time, like staying in your basement coding and maybe making a couple bucks off crappy video games you trick Steam's community into picking up.
Now I'm sure the next argument would be "their software/ToU/blah-de-bleepy-bloo". No court of law would even consider holding up any clause that says "A-bloo-bloo-bloo this person made me sad and therefore I reserve the right to deny them service". They'd probably hold up a clause that says "dude, if you come by my house with a knife I'll prolly not serve you anymore 'cause damn". Buuuuut that's not what happened here.
tl;dr What this customer did was wrong, and you'd have to actually be retarded to not acknowledge that. However, customer service means "customer service" and not "service so long as you appease me."