|There are a couple of things you can do, and they are not difficult.|
Switch your network to wpa, or better, wpa2 encryption, and then come up with a decent key of say, 12 characters. That will be practically impossible to crack.
If you really want to lock it down, enable access by mac address only. You may need to do an ipconfig /all to get the MA of you machine's adapter. Then put it in the router and exclude all others. That will be the end of the story.