Enter the router wifi setup go to the security settings and set a password. Its very easy! Once you take care of that and save settings any freeloaders will be kicked and a password will be required for them to connect.
1)Open the web browser that you use: Firefox,Google Chrome,IE,or Opera and type in the IP address for your router,and press the Enter key.
http://www.routeripaddress.com/ < Router ip addresses can be found here
2)A window will open and then type in your routers default username and password,and press the Enter key again
http://www.routerpasswords.com/ < Router usernames and passwords found here
3)Navigate to the Wireless Security\Network Security Settings Tab and open it to locate or change your WEP or WPA Security Passphrase. Use WPA because it provides better security.
4)Write this Security Passphrase down and put it in a safe place to access when needed.