Help with refund?

Long Version: I was at school on Steam, just seeing what's on promotion and talking to my friends. I closed it, my FaceBook, and my g-mail. Before i got away, it was only in sync, with doesn't take a long time since there's no game on my school PC. So then i came home and saw that by some reason Fortix was on my invetory and my account dropped from R$1,79 to R$0,10. I already changed the password of my Steam and my e-mail for security. One of my classmates then came laughing at me saying: "Hey dude, how do ya like Fortix?", i almost punched him. He din't gave the money that he owns me and now i'm stuck of a game that i already have. So here's my question: How do i get my money back? If there's not a certain way, i will giveaway it. If you don't trust me, here's proof:

"Date confirmed:
Wed Oct 23 10:25:06 2013."

I study from 7:00 to 17:00 and my intervals goes like this: 9:30 - 9:50, 12:20-13:20(Lunch time) and 15:00-15:20. Yeah, NOT ME. You could say that i bought from my cellphone, but not, it was Portuguese class, if i want to get to the internet i would have to use my 3G network since there's no wi-fi signal in the 4th floor of my school. If i used the 3G i will come on my account, but i am most sure that WILL NOT, because i DIN'T!

I don't even need to prove it, i wouldn't be begging for R$1,69 for no reason at all, no games besides Fortix can be bought with that amount. Also, i'm an active steam user, bought over +400 products from it, i have no VAC ban or any ban whatsoever and i'm a trusted trader (Steam profile for + rep and also steam trades). So yeah, i really am not kidding. I've done giveaways here also, so i'm not the one that would beg for games. I like to have many different games. But i sure don't like to have an extra copy of a game that i already HAVE. So, that's about it. Some dude found my account open, bought Fortix as an inventory gift and then logged out. I am *** mad right now and searching for the culprit. The purchase time gave me an useful tip: The launch time of the 1st year is from 11:30-12:30. So it has to be someone from it. And someone that does know Steam (i live in Brazil, don't expect that much). I have at least three peoples in my list, and i'm going to hurt them to spit the truth SO MUCH.

Thank you all for reading, and perhaps, being mad at my side. I really know this sucks, but i need help of you guys now: How do i get a refund? QUICKLY. I know Steam Support, but last ticket that i need to be answered urgently was simply ignored, my luck was that i could solve the problem alone (it was when "boleto bancário" first came into Brazil).

Short Version: TL;DR, some dick found my account open by somewat reason and bought Fortix as i gift. How do i get a refund?