How do I find out which program is using internet and how much?

• NetHogs may help (sudo apt-get install nethogs)
• Run with sudo nethogs <interface> (interface can be eth0, wlan0, etc.), and observe it for a while:

• Then use htop (install if necessary) to "graphically" kill the process. You will probably get better results by telling us which process it is and then disabling it, instead of swatting it like a fly every time it pops up ;)

Maybe IPTraf can help you.
I don't use it, but it seems to do what you need.