Maybe you should take a look at the wireless security settings on your router. It sounds to me like someone is using your network.
If you're using WEP encryption, don't. Use WPA2, or at the very least WPA. Have a nice, long password. I used to use this site to generate random passwords:
https://www.grc.com/passwords.htm . Don't forget to save it somewhere so you can copy and paste it to new devices.