It appears that my ISP has found a way to exceed my hi-speed bandwidth by sending garbage packages. Their customer care confirmed that even if just the router is switched on, it's going to keep adding up to the data-transfer and may even reach up to 20-25 GB over a period of two days! (No exaggeration – those were exactly their words)
Now, besides dragging the ISP to court for fraud, the only way to keep my data-transfer to a sane limit seems to be to switch off my router.
Is there a way to stop the data transfer between my router and ISP without switching off my router?