Wifi with a secure password, but yet another computer is linked

I have wifi with a secure password, but yet another computer is linked to mine! Only I am paying the bill. No matter how many times I have it worked on, it always comes back just like it was. I cannot get rid of it. I have even suspected the cable guy connecting it up this way. Can anyone give me feedback to help. No one will believe me. They think I am paranoid. Believe me I'm not!