One of my neighbours hacked the password of my router and he uses my limited internet package. I change the wifi SSID almost daily, but he can hack it easily. 
Today, he changed the SSID to a hate speech "insult". 

How can I stop him? I need a quick and powerful solution. Is there any easy-to-use software that protects my wifi?

I have an idea but I don't know how to do it. Sometimes my mobile (smart phone) finds a wifi network that does not have a password. So, I can connect to it easily. When I access the internet, all websites are unavailable. And I can not surf any webpage. How to do something like that?

Edit: I'm Using WPA/WPA2 PSK

There are two different passwords that access different functions.  If an attacker has the admin password, then he / she can change the SSID, WiFi password, and any other settings on the WiFi router.

To fix:  ensure your WiFi security setting is WPA or WPA2.  Then change the WiFi password to a long one (at least 12 characters, more is better) with special characters and numbers (such as #, $ %, !, 1, 6, see for example Is there any point in using 'strong' passwords?).  Also, make sure the admin password on the WiFi router is changed from the factory default.  This admin password is different than the WiFi password.  It should also be a long complicated password, but do NOT make it the same as the WiFi password.  The WiFi password is the one you give to friends and family to access your WiFi.  The admin password should be kept with you only, or people you REALLY trust, as it can be used to change WiFi settings.  Once this is done, change the SSID back to one you like.

Also, make sure to disable the feature called Wi-Fi Protected Setup (WPS).  See http://www.howtogeek.com/176124/wi-fi-protected-setup-wps-is-insecure-heres-why-you-should-disable-it/ for details on why WPS is not recommended.

If the attacker is still able to change the SSID and any passwords, your system is more deeply compromised and I would recommend contacting a computer expert or store who can help you clean your system.  They can also give you advice on if there is anything local law enforcement can do, as your attacker is likely committing a crime.
