Can someone wirelessly/remotely use my access point?

My family has had HughesNet for about 3 years now and it always seems as if our "download limit" is being exceeded when we really didn't download anything. Until Comcast gets to where we are (there is a small subdivision across the street, but we are pretty much in the country), there is no other hope for internet. I was just wondering if the guy who installed everything for us could be using our internet remotely. When he was installing the modem and router, he put his personal phone number as the key to our access point. Ever since I had found out that it was HIS personal number I wanted it changed, but we never did change it. I honestly believe that the culprit is really HughesNet with them just ripping us off, but what do you think? It is really frustrating. Also, as a little side note for those that play Minecraft. When the game is loading chunks, is that downloading anything?