The title kinda says it all. I uploaded some code once, but now i can't upload anything without it running the code. I tried shorting the RST and GND pin, but then the board's serial port won't show up on the ports menu.
Googling didn't really help me any more than that. 
Is there some way i can prevent it from running or just clear the memory?
Also: Please just request more info if needed.
It's not clear to me what you mean by "i can't upload anything without it running the code". Please give a more detailed description of what the problem is.
Double-tap reset (or connect reset and ground, then disconnect, reconnenct, disconnect - to simulate quickly pressing reset twice, if there isn't a reset button) to make it stay in the bootloader. Then upload clean code to it.
This happens when you upload code that breaks badly enough (usually via writing off the end of an array) that the USB functionality stops working.